Zeros made their debut alongside Kihunters in Super Metroid, appearing in the swampy depths of Brinstar on Planet Zebes. It was not until Metroid Fusion that it became clear that Zeros are larval Kihunters, with their metamorphic life cycle displayed over the course of the game; after Zazabi's defeat, all Zeros aboard the BSL Station enter their pupal stage and form indestructible cocoons, later emerging as fully-grown adult Kihunters. In Metroid: Other M (the source of their Smash design), Zeros were cloned aboard the Bottle Ship, where many Zeros could be found in the nest of a King Kihunter in the Biosphere.
In Super Metroid and Metroid Fusion, Zeros slowly crawl along floors, walls, ceilings, and other surfaces. In contrast with their dangerous adult forms, Zeros are typically passive and generally ignore Samus Aran, but they can damage her if she makes contact with their spiky bodies. In Other M, Zeros behave more aggressively, actively lunging at Samus and spitting webbing at her. In this game, Zeros' hard exoskeletons are impervious to all weapons except for Bombs, Plasma Beam, and the Screw Attack.
While Zeros do not make any appearance in their larval form, they are mentioned in the trophy description for the adult Kihunter.
As a stage hazard
Zeros, along with FG II-Graham units and Joulions, appear as stage hazards on Pyrosphere. The weighty Zeros drop down onto the stage and then crawl slowly on the floor. Zeros ignore fighters in a similar manner to their appearances in 2D Metroid games, but they do not cause damage on contact. If picked up by a fighter, their weight makes the fighter more difficult to launch. They can also be thrown at opponents or at Ridley to inflict damage.
Zeros do not appear when the stage is set to Ω form.