Sewer-Gator

Everyone knows there's gators in them sewers. You just better hope they stay there.

— Flo-Man, Sewer Explorer & Gator Hunter

Built to accommodate billions, the sewers of the Megacorpolis are an enormous labyrinth that even the minotaur would get lost in... And if he did, he'd probably get eaten by a Sewer Gator.
When the world collectively evacuated to the Megacity to escape what seemed near certain apocalyptic doom (don't worry, things got better), they brought animals with them. Entire zoos were airlifted out of flooding cities, and people brought pets, companions, even wild animals along with them - including a fair share of alligators. Since then, many have either escaped or followed the time-honored tradition of just being abandoned when they grow up to be huge, scaly killing-machines, and made their ways into the sewers.
Every year, countless tons of the worst kind of waste mingle in the cesspits beneath the city. Along with that, corporations dump industrial waste, chemicals, and failed experiments down the drain together, while Megacorpolis citizens get rid of everything from defect Java to hard drugs by flushing their toilet.

by NBCNews (And Dhelian)

Sewer Gators wallow in the filth and been changed by the unholy mix, because a regular gator wasn't bad enough.

Urban Legend

Well, myths don't usually-

— Sewer Maintenance Worker, last transmission.

Sewer Gators were once American Alligators, but thanks to their new home, they've become something else entirely. Horribly mutated, Sewer Gators range from relatively small (all the better to creep up your drainage pipes) to huge terrors that will haunt your dreams forever. While they have the same general form as their genetic ancestors (four legs, scales, big mouth, pointy teeth), Sewer Gators display a remarkable variety on that basic template.

by Evolve

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Everything from tentacles to eyes bursting out between scales or opposable thumbs.
If it's been in one of your nightmares, it's probably growing out of some SewerGator, somewhere.

Whether through the whims of fate or the intervention of nefarious corporate experimentation, a few common strains of the Sewer Gator have emerged to populate both the images flashing when some unfortunate maintenance workers close their eyes and popular media.

The largest Sewer Gator on record was named Big Ben after Fancy Ben, as they were both the same size, weight, and degree of radioactive.

Sewer Gators are highly aggressive, with many souped up 24/7 on a cocktail of discarded drugs and industrial waste. They'll chase anything with a pulse and prone to violent bouts of destruction when they're not busy chomping down on some poor bastard's skull. They hunt much like regular gators, lying submerged in puddles of half-coagulated filth before going on a murderous, drug-fueled rampage.
When food grows scarce in the sewers, they climb out of manholes and storm drains to look for something/someone to eat in the busy streets of Megacorpolis.

Sewers Suck

Sometimes, food comes down to them. The Megacorpolis sewers require constant maintenance, and although Cybiders could do the job with less risk and loss of human life, the profit margins would be worse. Cybiders and other robotic workers are considered much too valuable to risk down there.

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In addition, the rise of urban exploring as a hobby adds variety to the Sewer Gator's diet. Corporations even set up guided tours down through the "majestic underworld of Megacorpolis", with tour guides usually armed with flamethrowers.
Others venture down on their own or in small groups, live-streaming their experience to the surface. Fake encounters with Sewer Gators to drum up views are pretty common, so no one is more surprised than the streamers when one actually shows up.

by NuclearBeast

Monster Fight!

There's been plenty of attempts to deal with the Sewer Gators, and it's gone about as well as you might expect. At one point, intelligent Cybiders were released, but they ended up going rogue and more than likely responsible for the appearance of Cybergators, barring any corporate shenaniganery.

by Resident Evil 7

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The next idea being floated around is to release some sort of bio-engineered horror down into the sewers and let them duke it out, which couldn't possibly go wrong. So far, it's being held up as the corporations involved argue over movie and/or streaming rights.

Kingdom of Abandoned Pets

Alligators weren't the only pets to get unceremoniously abandoned when their owners couldn't be arsed anymore. Everything from snakes to goldfish to at least one great white shark has some way or another made its way down into the muck beneath the city, forming a vibrant eco-system of heart-break and horrible mutation.

by DailyMail

It keeps going to this day, kept steady by a diet of sewer maintenance workers, urban explorers, and fresh pets. Some animals have even achieved a sort of stable population, adapting to feed on the gruel of filth.
The circle of life is a beautiful thing, or something.

And yes, that shark is still down there.

Gator Hunting

There's a standing bounty on Sewer Gators, typically going up as the population of Gators do. This has lead to the predictable practice of deliberately feeding populations of Sewer Gators in a sector to make sure bounties are both high and plentiful.
To everyone's surprise and dismay, this sometimes goes horrible wrong, as with the Gator Tide of 2088.

Common Strains

Tyrants

Tyrant Sewer Gators are the largest of the Sewer Gator kind, sometimes bigger than a truck, and more than capable of eating one. Commonly featured in video games as bosses.

Slithering

Slitherings snouts peel open into five different teeth-lined nightmares, after which point most people stop paying much attention and start running.

Of course, the sewers of the Megacorpolis are vast. There are no telling how many Gators that might be down there, something a particularly hysterical strain of Doomies is all too happy to remind everyone of.

Javalligator

Faster than a speeding bullet and twice as likely to kill you, Javalligators are Sewer Gator who suffer from and have been mutated by late-stages of Javamania.

Cybergator

Thought to be nothing more than an urban legend made up to trick gullible urban explorers, Cybergators are very popular in media. They are less popular with the people who have encountered them, though.

by Rare Ltd

Origin Story Movie

There's some dispute over what came first: the Sewer Gators or the movies about them. It is probably only coincidence that the common Sewer Gator looks remarkably similar to the antagonist featured in the movies with the same name and probably not a PR stunt gone horribly wrong.

Probably.

Markatable Mystery

Between illegal excavation, secret corporate facilities, and malcontent hide-outs, there's really no telling how much the Megacity sewers still resemble the blueprints. As long as the toilets flush, most people in the city don't give it any further thought, though conspiracy theorists love to debate about 'What Really Goes On'.
Corporations are happy to profit from the mystery, with a whole industry dedicated to selling "mysterious artifacts" found within the depths of the sewers - in reality, printed goods fresh from the factory, which is probably to the benefit of everyone.

Some claim that the sewers have become so vast and tangled that space has collapsed on itself, and the deepest parts of the sewer lead to another subterranean world or hollow earth, separate from the Megacity's reality.
Which, admittedly, sounds pretty unlikely.