Zoomer

Zoomers are semi-spherical mollusks with spikes along their backs. They originally only appeared in two colors: yellow and red, and were called Geemers starting in Super Metroid, though the Metroid Prime series has since reinterpreted them as two separate species, with Zoomers being the weaker of the two. Zoomers are the first creatures ever encountered in the series. Their intellect is limited to walking in set patterns along the terrain, meaning that Samus Aran must accidentally run or jump into them in order to be harmed. The Zoomers located on Tallon IV have 18 eyes, while Geemers have only three of them.

The Metroid Prime Hunters logbook entry for the Zoomer states that it was the cause of a Flesh-Eating Bacteria on Zebes which caused the extinction of many native species.

There is also a talking red Zoomer in a story of the Nintendo Comics System on board the mechanical asteroid, Metroid. Another red Zoomer appeared on the cover of a Captain N: The Game Master issue.

Behavior
In all its incarnations, the simple-minded Zoomer is capable only of walking in set patterns. It will not directly attack Samus: she must instead accidentally touch one in order to take damage.

Related to Zoomers are the more resilient Geemers.

Metroid manual
"These fierce little creatures spray sticky fluid from their feet so they can move along the floor or up and down walls. There are two kinds: red zoomers and less powerful yellow zoomers."

Official Nintendo Player's Guide
"The first enemy Samus meets. He approaches by crawling along the floors and walls."

Official Metroid Prime website
"Zoomers anchor themselves to walls and other surfaces. Basic nerve centers located directly above the mandibles are able to detect nutrients which the Zoomers digest. Sharp spines protect them from casual predators, but the lack of a reinforced carapace makes them vulnerable to more persistent foes.

The Zoomer is protected by a spiny shell which covers a basic nerve center located directly above the mandibles."

Metroid: Zero Mission manual
"These fierce little creatures excrete a sticky liquid from their feet, allowing them to move up, down, and across floors, walls, and ceilings. There are two kinds of Zoomers: red ones and yellow ones. Red ones are much more powerful."

Official Metroid Zero Mission website
"SPECIMEN ID# X-7260 Wide leg-span and gripping nature of feet allows specimen to easily climb rock formations. Rock-crushing teeth suggests creature feeds on nutrients found in soil and ore. Exterior spikes are defensive in nature, but can be used to immobilize enemies. DANGER LEVEL: MODERATE."
 * "Pointed spikes"
 * "Gripping legs"
 * "Razor-sharp teeth"

Super Smash Bros. for Nintendo 3DS Trophy
"These creepy crawlers from the Metroid series cling to any surface, even the ceiling. The spikes on their backs are painful to the touch, so be careful around them. In Smash Run they'll travel on the floors or ceilings, so borrow a page from Samus's book and take them out from afar."

Trivia

 * Zoomers feature prominently in the Catch Mode of Tetris DS.
 * Zoomers somewhat resemble Spinies, from the Super Mario series.
 * Despite being called a mollusk, the Zoomer has legs and teeth in later Metroid titles, while most real mollusks possess neither. In the original Metroid game, Zoomers had tentacles for legs.
 * A Zoomer is depicted on the Metroid Trucker Cap.
 * Zoomers were intended for use in Metroid Prime 2: Echoes.
 * There is a slight glitch in Metroid Prime and the New Play Control! Metroid Prime: if a Zoomer is hit with a glancing shot from the Ice Beam, it will freeze in place, but the ice frames won't cover it.