Eel (Yoshi's Story)
From the Super Mario Wiki
(Redirected from Mr. Eel)
These enemies do nothing but mindlessly swim through the level, moving through walls and other obstacles; as a result, they do not pose much of a threat.
Eels always come in unlimited numbers, which can be very useful, as they can be eaten by a Baby Yoshi to restore life on its Smile Meter. When Yoshi eats a Eel, it gives him an egg but it is useless underwater.