Melon Bug

Melon Bugs are harmless, -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. Alternatively, they can be swallowed to create an egg.

Super Mario World 2: Yoshi's Island

 * Official Nintendo Player's Guide: These feisty hoppers transform from bug to melon and back again. Eat them anytime to make an egg.
 * Shogakukan guide: 近づくと、身を守ろうとして玉になる. 転がして敵をやつつけたりできるので便利. ("When you get close to it, it turns into a ball to protect itself. It is useful because you can roll it around and kill enemies with it.")

Yoshi's Island: Super Mario Advance 3

 * Shogakukan guide: 近づくと玉になって転がってくるが、ふれても痛くないし、タマゴで倒すこともできる. 食べてはき出せば、敵を倒しながら転がって行くぞ. (When you get close to it, it turns into a ball and rolls around, but it doesn't hurt when you touch it, and you can knock it down with an egg. If you eat it and then expel it, it will roll away while defeating your enemies.)

Trivia

 * Melon Bugs were originally designed for Super Donkey, a prototype game connected to Super Mario World 2: Yoshi's Island. An alternate version of their early sprites, with the final red shoes, also heavily resembles their appearance used in artwork.