'Zeros are enemies from the Metroid universe.
They are worm-like creatures 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. At first glance, they seem to be completely harmless, until Samus actually makes contact with them.
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.
They seem to have modified the Zeros, making them slightly larger and adding larger thorns that now cover most of the surface of their bodies.
After defeating the Zazabi boss, 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 (who are 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, Samus 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.
In Super Smash Bros. for Nintendo 3DS and Wii U
Zeros appear as enemies on the Pyrosphere in Super Smash Bros. for Wii U, alongside Ridley, Joulions and FG II-Graham Units. They fall from the air and move about the stage. Fighters can pick up the Zeros and use them to attack each other or to attack the Ridley stage boss.
"These arthropods with hard exoskeletons are the larval form of Kihunters, a type of enemy from the Metroid series. In this game, they appear on the Pyrosphere stage, where they fall from above and then move about slowly. Throw them to attack your opponents or Ridley, or keep hold of one to make yourself harder to launch."