The Ender Muncher

A mob that spawns on bodys of water on the end (a small, lonely island whit a lake), it has a big alligator-like head and a small body that just hangs down when in the water. They will attack anything in the water in a 10x10 area. Players can throw stone/cobblestone for them to chew into sand or concreat to be chewed back into powder form.

Also, this mob can live outside of water, but it would be slower than turtles.