Adapting D&D 5e for Pokémon (Work-In-Progress)

There have been a couple really great adaptations of D&D 5e for Pokémon. I would heartily recommend them!
This is less of a premade player's handbook and more of an on-the-fly set of conversion methods that you can use based on the regular Pokémon and Moves from the games. Bulbapedia is your best friend for easily finding those numbers.

Creating Your Trainer

Because most of the versatility of skills, abilities, and "magic" is found in Pokémon and not their trainers, your trainer character will be a little less independently powerful than a typical D&D character.

Origin

Time to mish-mash D&D Race and D&D Backgrounds together into one conglomeration that will serve as our trainer's origin.
Ability Scores
Increase one ability score by 2 and a different ability score by 1.
Save Proficiency
Choose 1 saving throw proficiency.
Skill Proficiencies
Choose 3 skill proficiencies.

For Pokémon settings, three new skills are added to the usual D&D list of options: Pokémon Handling (replaces Animal Handling), Pokémon Knowledge (INT), and Technology (INT).

Depending on your setting, you may also choose to remove Arcana (INT) and/or Religion (INT) from the available options.

Tool Proficiencies
Choose 1 tool proficiency. Recommended options:

Artist's Tools (choose a specific medium that is your specialty)

suggested options: 2D dry media (sketching, charcoal, pastels), 2D wet media (inks, watercolors, acrylic paints, oil paints), 3D dry media (sculpting or whittling), 3D wet media (pottery), fabric art (weaving, crochet, knitting, embroidery, etc)

Writer's Tools (same as Calligraphy Supplies + Forgery Kit)

Cook's Kit (can serve as a pared-down Herbalism Kit)

Tinker's Tools

Performance Kit (same as Disguise Kit)

your choice of musical instrument

your choice of common game (chess, checkers, dice, playing cards, etc)

Note: If you want to get funky with it and have Definitely Inhuman player characters, check out the Build-a-Body Workshop.

Class: Trainer

For the sake of this document, we're going to consider anyone who works extensively with Pokémon to be a Pokémon Trainer in terms of class mechanics.
To summarize for now, being a trainer allows you to catch, befriend, care for, and battle with Pokémon. You need a valid Trainer's License to carry more than 2 Pokémon with you at any time (unless you're working as a transporter, which has its own permit to bear) or to battle with other trainers in any setting. You have a maximum of 6 Pokémon you can carry with you at one time (again, unless you have a transporter's permit).
Becoming a level 1 Pokémon Trainer means you have received your Trainer's License and a starter Pokémon, as well as a localized PokéDex. (Depending on your setting, your PokéDex may be a complete printed guide, a haphazard collection of unfinished notes, an app for your Rotom phone, or a separate device.) You also gain proficiency in your choice of either Pokémon Handling (WIS) or Pokémon Knowledge (INT).
You start the game with 5 PokéBalls, 3 Potions, and 1 Town Map of your hometown and local area.
Trainer Specs
At level 3, you can choose a specialization for your trainer that will provide a target set of benefits to help you reach your goals. You can also choose a local Pokémon Professor (or equivalent authority) who will sponsor and guide your training, research, and caretaking of your Pokémon.
Options:

Ace Trainer (future Pokémon Master) - bonuses to Pokémon battling

Jack-of-all-Trades

Caretaker (Nurse+Pokébreeder)

Researcher (future Pokémon Professor)

Collector - bonuses to Pokémon catching

Type Specialist (future Gym Leader)

Ranger

Pokémon Mechanics

Determining Pokémon Stats

For today's example, we're going to be using one of my favorite Pokémon, Mawile. She's a Steel/Fairy type from Generation III.

Stat to Stat Mapping
HP -> Constitution
Attack -> Strength
Defense -> Armor Class
Special Attack -> INT or CHA
Special Defense -> WIS or INT
Speed -> Dexterity
left over: 1 of INT/WIS/CHA

Mawile Stats
Constitution: 10 (+0)
Strength: 17 (+3)
Natural Armor: 17 (+3)
Charisma: 11 (+0)
Wisdom: 11 (+0)
Dexterity: 10 (+0)
Intelligence: 7 (-2)

Armor Class: 10 + Dexterity modifier + Natural Armor modifier.
In this case, Mawile's AC would be 13 (10 + 0 + 3).

Let's take a look at her game stats from Bulbapedia. She's a Pokémon who doesn't evolve, and her base stats are middling at best. Let's work out what her D&D stats would be as a level 1 Pokémon.
First, which stat is which? HP is CON, ATK is STR, and Speed is DEX. DEF would translate to the Natural Armor bonus, which can be treated as a "normal" stat to get the modifier, which is then added to the Armor Class (which equals 10 + Dexterity modifier).
It gets a little more unpredictable with the Special -> mental stat set, though. I'd call Special Attack either INT or CHA and Special Defense either WIS or INT, based on the flavor of the Pokemon and its Type(s).
Now, Mawile is more about bite force and savagery than precision and accuracy, so I'd pick her Special Attack as CHA and say her Special Defense is WIS. That leaves INT without a corresponding stat, but we'll get to that in a moment.
Let's talk numbers. Double the Pokemon's base stat, then divide by 10. That means Mawile's 50 HP becomes 100, then 10, which makes sense as a mediocre CON stat. Her ATK and DEF are both 85, which doubles to 170 and then divides to 17. That's a +3 modifier for her STR and her AC. Her DEX is the same as her CON, and both her CHA and WIS would be 11, giving her a +0 modifier.
Now, what about her missing INT? I'd propose a default INT for most non-Psychic-Type Pokemon should be 6 or 7, which is a -2 modifier. In D&D, that's a pretty low intelligence for a person but much higher than a normal animal, which fits where most Pokemon seem to be. Fairy Types tend to be a little smarter than average, so let's put Mawile at a 7. I'd say 8 would be a reasonable score for a Psychic Type or a Pokemon noted as particularly intelligent, like Lapras.

That gives us a pretty good stat set for a level 1 Mawile, but we're missing movement speed, hit dice, starting HP, and proficiency bonus. Proficiency bonus can be the same as a player character, starting at +2 at level 1.
Hit dice are a little more complicated, but I would recommend basing them on the Pokemon's size; Tiny Pokemon would have d4s, Small Pokemon would have d6s, Medium Pokemon would have d8s, Large Pokemon would have d10s, and anything bigger would have d12s. The number of hit dice is equal to the Pokemon's level + 1, so a level 1 Mawile would have 2d6, estimating her size as Small since she's about 2' tall. (D&D size classes can be compared with a Pokemon's size measurement from Bulbapedia.)
For calculating HP at level 1, I'd use the same method as for a player character: let the first hit die count as a maximum roll, then use the average or roll for the rest of the hit dice. For Mawile, that'd put her HP at 10 (maximum roll of 6 + an average of 4 + a Constitution bonus of 0) - that's pretty comparable to a level 1 creature without a high CON.
That just leaves us with movement speed, which doesn't have an easy conversion. Speed in the Pokemon games is related to action speed, not movement speed. Honestly, this is where I would just eyeball it; Mawile's a Small biped without any speed-boosting traits, so I'd put her at 20 feet per round. She has no reason to be particularly good at swimming or climbing, so those speeds would be standardized as half her normal movement speed, or 10 feet per round.
Guesstimating movement speed for each type of movement is simply not going to be a straightforward equation. Pikachu, a Small quadruped, would have a movement speed of 30 or 35', and I'd call him proficient in climbing, so his walking and climbing speed would be 30' and his swim speed would be 15'. Slowpoke, a Medium quadruped, would be 15 or 20' per round, but his swim speed would be faster, maybe 25' per round. Ponyta is a Medium quadruped who would probably be a speedy 40' per round, while similarly-shaped Mudbray might only be 25' per round. Looking up comparable creature stat blocks in D&D 5e would probably be your best bet for a quick number, especially if you're not familiar with the Pokemon in question.
That doesn't even get into the levitating-but-not-flying Pokemon, the teleports-as-part-of-their-movement Pokemon, the has-a-swim-speed-but-no-walk-speed Pokemon... You get the point. It's messy.

Capturing Pokémon

FINDING A POKEMON...

1. Choose your starting location on map's edge, then roll 1d20 + your proficiency bonus.

2. If you roll 12 or higher, there is a sound of a Pokemon in that direction.

If you roll under 12, you hear nothing. Move to an adjacent space of your choice.

3. If you roll a 20 or higher, roll 1d20 again; you hear two pokemon in different directions.

You can choose either direction and may visit both in whichever order.

4. If you roll 2 natural 20s, whatever you find in 1 of the 2 locations is Shiny (DM's choice).

5. Move to the location of the Pokemon sound. To determine which Pokemon you find, use this to generate a biome-appropriate list and roll 1d10 to see which from the list appears.

THROWING THE BALL...

Roll 1d20 + your PROF to hit pokemon with a ball.

Tiny or smaller Pokemon: DC 11 (same square) / 13 (adjacent)

Small or Medium Pokemon: DC 9 (same) / 11 (adjacent)

Larger or larger Pokemon: DC 7 (same) / 9 (adjacent)

If Pokemon is unaware (asleep, surprised, etc), -2 DC to hit.

On a nat20, pokemon makes escape check with disadv.

THE BALL SHAKES...

Pokemon makes an escape check by rolling 1d20.

Escape DC equals 6...

+ Pokemon's Catch Rate bonus**

+ how many quarters of their HP total are missing (0-4)

+1 per status condition (paralyzed, etc)

+4 if Great Ball OR +8 if Ultra Ball

If the Pokemon escapes with >50% health, it keeps fighting.

If the Pokemon escapes after being surprised, it may flee.

**To calculate the Catch Rate bonus, look up the normal catch rate of a pokemon on Bulbapedia. Ex. Pidgey has a catch rate of 255 (43.9%), which is the maximum catch rate, and Mawile has a catch rate of 45 (11.9%), while a Legendary Pokemon like Lugia has a catch rate of 3 (1.6%). Take the percentage, divide by 10, round down into a whole number, then double that. (Ex. Pidgey's 44% would become 4.4% -> 4 -> 4 * 2 = 8, Mawile's 12% would become 1 * 2 = 2, and Lugia's 2% would become 0 * 2 = 0.) If you're throwing a normal pokeball (no modifiers) at a pokemon with full health (no modifiers) and no status conditions (still no modifiers), you're going to add 6 to the catch rate bonus to get the pokemon's escape DC. For Mawile in our example, her escape DC would be 8, Pidgey's escape DC would be 14, and Lugia's would be 6. (I would make a house rule that Legendaries and Mythicals cannot be caught with a normal ball, but to each their own.)
Of course, if you're the DM and this sounds way too complex, skip the catch rate modifier and just use a static escape DC.

Caring for your Pokémon

Bonding with your Pokemon

This is optional, but a Bond system can represent your relationship with your Pokemon and give a trained Pokemon that extra boost that comes from having a trainer working consistently with it. A positive Bond bonus can enhance your battling prowess and your Pokemon's skill checks and saving throws; a negative Bond bonus worsens your odds and can put you at risk of your Pokemon disobeying or even abandoning you.
Freshly-caught Pokemon start at Bond 0, but starter pokemon or wild pokemon who willingly chose you can start at Bond +1. Stolen Pokemon start at Bond -1.
A positive Bond bonus cannot be higher than the trainer's proficiency bonus; a negative Bond penalty can go to -3 before the pokemon tries to escape/abandon you.
Major events can change a Bond rating. Save your Pokemon's life at risk of your own? Gain a Bond point. Ignore your Pokemon's warnings and use them as cannon fodder? Lose a Bond point.
Depending on the day-to-day treatment of your Pokemon, your Bond level may rise or fall over time at the DM's discretion. (If you make a mistake and lose a Bond point, you can regain it with enough TLC during downtime; likewise though, neglect your Pokemon consistently and you'll lose a Bond point without a major catalyzing incident.)

Healing your Pokemon

This is a bit flexible. You could keep it very D&D with standard comestible Potions that work on Pokemon the same as player characters, ignore Revives as a separate mechanic, let Pokemon use their hit dice to heal on a short rest, and everyone recovers fully from a long rest.
On the other hand, if you'd like to be more on the Pokeverse side of things, you can use spray Potions to recover HP if your Pokemon's conscious, but need to use a Revive to rouse them from unconsciousness (or take them to a Pokecenter). Instead of using their hit dice during a short rest to heal, maybe you need to feed them in order to trigger that kind of healing. You could even say that Pokemon don't recover on a long rest, but only at a Pokecenter, or maybe they just don't recover any spent hit dice until they visit a Pokecenter.
Or y'know, mix it up. Regular D&D Potions work for people and Pokemon both. Hit dice can be used during a short rest, and everyone recovers on a long rest. Pokecenters can provide a long rest's benefits to your Pokemon over the course of a short rest. But if your Pokemon gets knocked unconscious (drops to 0 HP), it automatically goes back to its pokeball and cannot be retrieved until healed fully at a Pokecenter. (This means Pokemon do not risk death or have to make death saving throws, but can also leave you very dangerously stranded.)

Pokémon Combat

Combat generally follows regular D&D combat with initiative, rounds, attacks of opportunity, attack and damage rolls, AC and HP.
Specifically for Pokémon settings, in a 1v1 trainer battle, the trainers roll initiative and all of their participating Pokémon act on their turn (such as for double battles).
When Pokémon are out of their balls and can react independently to an unexpected situation, they will roll initiative independent of their trainer and use their own stats. Pokémon will usually defend themselves and take the Dodge action unless commanded otherwise, though some Pokémon may be aggressive enough to automatically attack any threat.

Commanding your Pokémon

Free action: commanding your Pokémon to move on their turn(s). You can always direct them to go towards or away from a general point.
Action: commanding your primary Pokémon to act in battle. This includes movement + Pokémon action + Pokémon bonus action. Your Pokémon will add their Bond bonus to their attack rolls, skill checks, and saving throws while it's your primary focus.
Bonus action: commanding a secondary Pokémon to act in battle. This includes movement + Pokémon action OR Pokémon bonus action. This Pokémon will not add their Bond bonus to their rolls, only their own proficiency bonus.
Reaction: commanding any of your Pokémon to take their reaction (when it's not a held action). Any Pokémon can take an attack of opportunity as per standard D&D rules without a command from you.
Movement: you can always move up to your full speed (as allowed by terrain/obstacles) on your turn, regardless of what commands you give to Pokémon.
In addition to the above, you as the trainer can also make skill checks like Perception or Investigation as an action (and use your bonus action to command one of your Pokémon without the Bond bonus), or you can use objects (including Potions and PokéBalls) as a bonus action (and still use your action to command your Pokémon).
But what if you have more than 2 Pokémon in battle at a time???
You can use your bonus action to command them as a group to do the same thing, e.g. Attack That Guy or Protect That Guy. It will be up to each Pokémon what ability or Move they use to fulfill that request, and any Pokémon who has a negative Bond bonus may ignore your command.

Types and Damages

Easy peasy lemon squeezy. Super Effective damage is doubled. Not Very Effective is halved (round down).
If you need to know what your Pokémon's weaknesses and resistances are, refer to Bulbapedia.
If you're immune to a type then you're immune to its damage, not its status moves.
If the move gets STAB, add the Pokémon's proficiency bonus to the damage.

Pokémon Moves

Let's talk about how to adapt game-canon Pokémon moves to D&D 5e on the fly, using a couple common attacks: Crunch and Dazzling Gleam.
Which stat?
If the move is physical, choose either STR or DEX as best befits the move. If the move is special, choose INT, WIS, or CHA as best befits the move.
For example, Crunch is a move based on crushing something in your jaws or talons, so it would be Strength-based. Fury Swipes, a multi-hit attack, would be all about speed and could use Dexterity instead.
For Special moves, use your Pokemon's Special Attack stat. For Mawile, that'd be Charisma (as we established in an earlier section for a clever Psychic Type like Kadabra, it would be Intelligence; for an intuitive Grass Type like Bulbasaur, you could choose Wisdom.
Roll for attack
Roll 1d20 + your Pokémon's proficiency bonus to hit (vs the target's AC). If you are actively commanding your Pokémon in battle, you can add your Bond bonus to the attack roll.
Accuracy
Take the original move's % accuracy, subtract from 100, and divide by 10 - then subtract that final number from your attack roll. This is less terrifying than it sounds - if you have 80% accuracy, that means your attack roll has -2.
Damage dice
Although I am extremely tempted to say "roll n d4s based on damage", let's think about this. Add the relevant stat bonus (the one you rolled to hit) as damage to the dice:

Canonical damage 10-35: that's a d4.

Canonical damage 40-55: that's a d6.

Canonical damage 60-70: that's a d8.

Canonical damage 75-85: that's a d10.

Canonical damage 90-100: that's a d12.

Canonical damage 100+: 1d12 plus whatever die equates to the leftovers. (E.g. 110 damage = 1d12 (from 100) and 1d4 (from 10).)

If you're running moves like this, make sure to treat them like cantrips and add an additional damage die when the Pokémon reaches certain levels. A level one Eevee using Swift would deal 1d6+CHA Normal damage, while a level 12 Eevee using Swift would deal 3d6+CHA Normal damage.
Status moves
Instead of subtracting move accuracy from your attack roll, add it to the target's saving throw. (If it's an accuracy of 90, they get a +1 to their saving throw to avoid the move.)
To determine what kind of save to make, consider what the move is trying to inflict.

Sleep? CON save.

Paralysis? CON or STR save.

Poison? CON save.

Confusion? WIS or INT save.

Burn? DEX save.

Freeze? CON or STR save.

Flinch? CON or WIS save.

Let's translate these to D&D mechanics...

Sleep and Frozen: Target is incapacitated falls prone. (If flying or climbing, will fall to the ground.) At the end of each of the target's turn, the target can repeat the saving throw (without the accuracy bonus) to try to wake up. Each time the target takes damage, it makes the saving throw with advantage (without the accuracy bonus). If target is Frozen, being damaged by a Fire-type move guarantees it will unfreeze.

Paralysis and Flinch: Target rolls a d4. On a 1, the target cannot take its action, bonus action, or movement. For Paralysis, at the end of its turn, the target can repeat the saving throw (without accuracy bonus). (Flinch only lasts 1 turn.)

Poison and Burn: Target takes damage at the end of its turn for a number of turns equal to your Pokémon's proficiency bonus. If the target douses itself in water or loose dirt while Burned, or if a Water-type move is used on it, the Burn is removed.

Confusion: Target rolls a d4. On a 1, the target uses its action to make an attack on itself or its allies (DM's choice). At the end of its turn, the target can repeat the saving throw (without accuracy bonus).

Move Power Points (PP)

Treat each move's PP as spell slots that can only be used for that particular move. If Hydro Pump has 5 PP and Crunch has 10 PP, your pokemon can only use Hydro Pump 5 times before needing a long rest or a full recovery at a Pokecenter, even if they still have PP left for other moves.

Example Encounter

We have our level 1 Mawile, who has a Bond of +1, and we're scouting for wild Pokemon.
Mawile begins attacking a wild level 1 Sableye. Sableye has the same total base stats as Mawile, but differently distributed; its AC is 12.
Mawile's Turn

Mawile uses Crunch. Crunch is a Dark Physical move that has a Power of 80 and an Accuracy of 100.

Mawile rolls to attack, using her Strength bonus. She rolls an 8, then adds her STR (+3) and her proficiency bonus (+2) and her Bond bonus (+1). Crunch has a 100 Accuracy, so it doesn't reduce the roll. That's a total of 14 to hit, which beats Sableye's AC of 12.

Crunch's Power is 80, so its damage die is a d10. She's only a level 1, so it's only 1d10, plus her Strength modifier (+3). She rolls her d10 and gets a 3, making a total of 6 Dark damage to Sableye.

Sableye is a Dark and Ghost Type. Dark Types have resistance to Dark moves, but Ghost types are vulnerable to Dark moves, so the type effectiveness cancels each other out; Sable takes the full 6 points of damage.

Also, Crunch has a 20% chance of lowering the target's Defense (AC) for the rest of the battle. Sableye needs to roll a d10 and not get a 1 or 2, else his AC goes down by 1 point, from 12 to 11. (He rolls a 7, so he's fine.)

Sableye's Turn

Sableye is irritated, so he's going to use Fury Swipes. Fury Swipes is a Normal-type Physical move with a Power of 18 and an Accuracy of 80.

Sableye rolls to attack, using his Dexterity modifier. He rolls a 17, then adds his Dexterify modifier (+0) and his proficiency bonus (+2) for 19. Since Fury Swipes has an 80% Accuracy, that's a -2 to the roll, making the final total a 17 to hit. That still beats Mawile's AC of 13.

Fury Swipes' Power is 18, which makes its damage die a very sad d4. Sableye rolls a 3, plus his Dexterity modifier (+0), making a total of 3 Normal damage to Mawile.

Mawile is a Fairy and Steel type, so she's resistant to Normal damage and only takes half. We round down in these instances, so she only takes 1 point of damage.

Fury Swipes is a multiattack! Bulbapedia tells us that there's a 35% chance to hit twice, a 35% chance to hit 3 times, a 15% chance to hit 4 times, and a 15% chance to hit 5 times. That means 1-35 is two hits, 36-70 is three hits, 71-85 is four hits, and 86-100 is five hits. Sableye rolls a percentile die (or 1d100 if he has one) and gets 45. That's 3 hits!

Sableye rolls to attack for his second Fury Swipe. He gets a 4, which gets a +0 from his Dexterity modifier, a +2 from his proficiency modifier, and a -2 from the move's Accuracy. That's a miss.

Sableye rolls to attack for his third and final Fury Swipe. He gets a 13, +0 DEX, +2 PROF, -2 ACC, = 13 to hit. "Meets beats," so a 13 to hit beats Mawile's 13 AC!

Sableye rolls his 1d4 for damage and gets a 4! But Mawile's still resistant to Normal damage, so she only takes 2 points. Sableye did a total of 3 points of damage on his turn.

Mawile's Turn

Mawile's trainer remembers that Sableye is weak to Fairy Type damage, and Mawile knows the move Draining Kiss, so she uses that next. Draining Kiss is a Special move with a Power of 50 and an Accuracy of 100.

Mawile rolls to attack, now using her Charisma bonus of +0. She rolls a 13, then adds her CHA bonus (+0), her proficiency bonus (+2), and her Bond bonus (+1). That's a total of 16 to hit, which beats Sableye's AC of 12.

Draining Kiss's Power is 50, which means its damage die is a d6. She rolls a 2, then adds her CHA modifier (+0) and her STAB bonus (+2, the same as her proficiency bonus), for a total of 4 Fairy damage.

Sableye is weak to Fairy damage, which doubles the amount of damage he takes to 8 damage. That's enough to knock out our erstwhile friend, but since this is a Trainer vs Wild battle, he will try to flee instead.

Also, Draining Kiss restores half the damage taken as HP to its user, so Mawile heals 4 HP (up to her max; this doesn't function as temp HP that can stack).

Mawile's trainer can either let Sableye go or use their reaction to throw a Pokeball before it vanishes from sight.

Mawile's Trainer's Reaction
...of course we're going to try to catch it. We have 3 empty normal Pokeballs with us, but only one shot.

To throw the ball, we roll 1d20 and add our proficiency bonus. We roll an 11 and add 2 for a total of 13.

Remember, the DC to hit the Pokemon with the ball is not the Pokemon's AC! Sableye is a little Pokemon and is actively fleeing, making the DC 13. Meets it beats it, so we hit!

Sableye rolls 1d20 to try to escape!

The DC to escape is 6 (default) + the Pokemon's catch rating bonus + how many quarters of their HP are missing (0-4). That means the DC is 6 + 2 + 4 = 12. Sableye must roll a 12 or higher on his d20 in order to escape the pokeball.

Sableye rolls an 18 and successfully escapes, breaking the pokeball in the process! He vanishes into the darkness, leaving our Mawile with 7 of 10 HP and us with only 2 pokeballs to our name.