Arachnus

Arachnus is a boss character in the Metroid series who first appeared in the Game Boy title Metroid II: Return of Samus. It is a multicolored creatured (mostly red, pink, and green) with a large shell for protection. Like the protagonist Samus Aran, it is able to roll up in a ball and move around attempting to harm the bounty hunter.

The main challenge of this fight is the fact that it's in its shell for quite some time, and you're only able to harm it when it's outside of it. Still, it was a fairly easy boss. Once you defeated the beast, Samus would obtain the Morph Ball spring ability.

Apparently Arachnus is a species rather than just a boss, seeing that another one of them was found in the Game Boy Advance game Metroid Fusion, where it yet again played the role of a boss, though this time under control of X Parasites, making it harder to defeat. Once you beat this creature in Fusion, you'd get the Morph Ball.