Zero

A Zero, also known as Zoro, is a worm-like creature found on Zebes. Later revealed to be infant Kihunters, they first appeared in Super Metroid, colored green, 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 actually makes contact with them, thus harming her with their bodies lightly-covered in thorns. Zeros have a 99% chance of dropping a Power Bomb Ammo and 1% chance of dropping an Energy Capsule when killed.

In Metroid Fusion, all Zeros Samus encounters have been infected and imitated by X Parasites. The X seem to have modified the Zeros, making them slightly larger and adding larger thorns that now cover most of the surface of their bodies. These Zero-X also have unnatural colors (red and blue) that differ greatly from their normal green tint; the blue variant is far more resistant than the weaker red type, and almost always release a Red X when killed. After defeating Zazabi, all Zeros on board the BSL Station harden their skins to an indestructible state. Soon afterwards, the Zeros enter a pupal stage; their cocoons, which are still indestructible, are visibly progressing towards an insect-like adult stage. Halfway through the game, the cocoons have hatched and Kihunters (also infected by X Parasites) are seen roaming around near the dried-up husks; this sequence of events was the first to reveal that Zeros are actually the infant/larval stage of the Space Pirate allies. Zeros appear in Metroid: Other M in their first 3D appearance. This time, they have a brown color scheme rather than the green, red, and blue seen in Super Metroid and Metroid Fusion. They also feature optical organs not seen in past games. The Zeros are immune to all weapons except for Bombs, Power Bombs, the Plasma Beam, and the Screw Attack. If they are shot by any other weapon repeatedly, they will quickly metamorphose into adult Kihunters within the space of a few seconds accompanied by a spray of a whitish fluid.

The Zeros of Metroid: Other M are first encountered in the Biological Experiment Floor. Looking around, she notices several Zeros and attempts to proceed without disturbing them. However, a loud screech caused by Little birdie puts the creatures into a state of stress, and several Kihunters rush in to defend them and attack Samus.

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

Trivia

 * While these creatures are indeed the larval stage of Kihunters, it is unknown whether Zeros possess any form of sentience/intelligence like the adults.
 * Zeros are curiously located far from any Kihunter in Super Metroid, the game where both creatures made their first appearance.
 * They may or may not be spawned from a King Kihunter.
 * In Super Metroid, the room directly before Spore Spawn's features both Kihunters and egg-like structures on the ceiling. These may be Kihunter eggs and thus would contain Zeros in development.
 * Strangely, Zeros are capable of molting into their adult Kihunter stage in a matter of seconds in Other M, while the infected variants and mimics seen in Fusion require a far longer amount of time. The Zeros onboard the Bottle Ship may have been artificially modified/enhanced by the scientists onboard in order to allow them to reach their adult stage more quickly, thus making the species into more efficient bioweapons. It is unknown why the X did not alter the Zero's DNA to make the molting faster, as they have been seen to alter DNA with Arachnus, Dessgeegas and others.
 * In Metroid Fusion, Zeros will travel noticeably faster as they receive more damage.