Horizonback Tortoise

Gargantuan Monstrosity - Unaligned
Armor Class: 17 (Natural Armor)
Hit Points: 227 (13d20+91)
Speed: 20ft
STR - 28, DEX - 3, CON - 25, WIS – 10, INT - 4, CHA - 5
Saving Throws: STR +12, CON +10
Damage Immunities: Poion
Condition Immunities: Poisoned
Senses: Darkvision 60ft, Passive Perception 10
Languages: Understands Goblin but can't speak
Challenge: 7
Traits
Amphibious - The tortoise can breathe air and water.
Massive Frame - The tortoise can carry up to 20,000 pounds of weight atop its shell, but moves at half speed if the weight exceeds 10,000 pounds. Medium or smaller creatures can move underneath the tortoise while it's not prone.
Actions
Bite - Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 28 (3d12 + 9) bludgeoning damage.
Shell Defense - Recharge 4 - The tortoise withdraws into its shell, falls prone, gains resistance to all damage except psychic, and gains a +5 bonus to AC. While the tortoise is in its shell, its speed is 0 and can't increase. The tortoise can emerge from its shell as an action, whereupon it is no longer prone.