Vicious Fish (5e Creature)
From D&D Wiki
Tiny monstrosity, lawful evil
Water Breathing. The vicious fish can breathe only underwater.
Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage.
Under the right circumstances, even small, nonviolent fish can become grotesque, blood-sucking fiends when exposed to the waters surrounding the brain of Cthulhu's lair. They distend with additional muscle, and sprout supernumerary eyes as their original ones become too opaque to see through.