Zero

A Zoro (also known as Zero) is a larva-like creature found on Zebes. They first appeared in Super Metroid, and were always seen crawling out of dried up skins, most likely from recent moltings. At first glance, they seem to be completely harmless, until Samus would actually make contact with them, thus somehow harming her. These have a 99% chance of dropping a Power Bomb and 1% chance of dropping an Energy Capsule when killed.

In Metroid Fusion, all Zoros have been infected and imitated by X Parasites, and have been subsequently modified by them. They seem to have gotten slightly larger, as well as gaining thorns that now cover the surface of their bodies. The Zoro X have also been given unnatural colors (red and blue) from their normal green tint. After defeating Zazabi, all Zoros on board the BSL Station harden their skins to an indestructible state. This could be seen as the beginning of the chrysalis stage. Later on, the Zoros' bodies, now cocoons, can be seen containing insect-like beings within them. Halfway through the game, the cocoons have hatched and Ki-Hunters (also containing X Parasites) are seen roaming around near the dried up bodies; this sequence of events reveals that Zoros are actually the infant/larval stage of the Space Pirate allies. However, because of the X Parasites' known ability to manipulate DNA, this metamorphosis cannot be considered with complete certainty as a natural evolution.

Creatures bearing great resemblance to Zoros are set to appear in Metroid: Other M. Whether or not they are the same creatures has yet to be confirmed. Also, Ki-Hunters seem to be returning as well; the game may shed light on the possible relations between Zoros and the Pirate allies. Their seems to be a confirmation of this relationship as a recent "Metroid: Other M" trailer shows Samus taking aim at a swarm of Ki-Hunters that descend upon her while a group of Zoros crawl around behind her.

Super Metroid Nintendo Player's Guide
"Destroying a Zero usually yields a Power Bomb."