Melon Bug

Melon Bugs are harmless, pill bug-like creatures that appear in Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3.

They bounce up and down, but when Yoshi gets too close, they roll up into balls, which resemble melons, hence their name. If Yoshi gets close enough, he can eat them and then spit them out, causing them to roll across the ground, destroying any enemies in their path.