Geemer

A Geemer is a "cousin" of the Zoomer which, in Metroid Prime and Metroid Prime Hunters, is invulnerable to the Power Beam. When attacked in this fashion, it defends itself by protruding spines. The only way to destroy a Geemer in Metroid Prime Hunters is to shoot it with any of the six sub-weapons or a missile, and in Metroid Prime, to shoot it with a Missile or the Plasma Beam.

Geemers seem to have appeared in Metroid Fusion, as there is an X variant of a creature which bears a striking resemblance to them and shares the same characteristics mentioned above (see image), and any physical differences between them may have very well been caused by the X.

In Metroid Prime 3: Corruption there are Geemers on Bryyo but no Zoomers.

Interestingly, Super Metroid's code refers to Geemers as Zoomers, indicating that the "new species" was an invention of the translation team. Several other creatures changed names between the original game and Super, but only Geemers emerged as a separate species alongside the original in subsequent games.

Super Metroid Manual
"Geemers crawl on floors and walls."

Metroid (1986 manga)
"This organism excretes a mucus from its legs while crawling on the rocks. The yellow Geemer can be destroyed with two shots from the normal beam. The red ones are destroyed with four."

Logbook Entries
Metroid Prime: "Wall-crawling mollusk with retractable spikes. The Geemer is an evolutionary offshoot of the Zoomer family. When threatened, it extends lethal spikes and retracts its head deep into its armored carapace."

Metroid Prime Hunters: "A small but dangerous creature that exposes sharp spikes on its back when threatened. This scavenger is known to forage through human refuse and is an infamous pest on ZEBES."

Metroid Prime 3: Corruption: "The spiked shell of a Geemer is rather durable and can only be destroyed by an explosive blast. While menacing in appearance, they are nonaggressive scavengers that feed on the waste of other creatures. This food source has led to them being spreaders of many types of diseases."

Artificial Intelligence
The AI routine is as follows: The Geemer begins by moving in either the left or right direction. When the Geemer encounters a vertical wall, it rotates by 90 degrees and walks up the wall it hit. Conversely, if the Geemer walks off of a ledge, it rotates -90° and walks down the wall. This allows Geemers to move in simple circles around a complex room or object.

There is a flaw in the Geemer AI in the original Metroid. If the object the Geemer is standing on is removed, the Geemer continually spins in place, as the AI is constantly rotating by -90°. This is fixed in Super Metroid, as the Geemer unceremoniously falls to the floor.

In Super Metroid, a Geemer of the ordinary blue/purple type is one of the first enemies encountered. It does minimal damage. When Samus Aran fires a Super Missile at a wall or ceiling, the resulting shockwave causes all Geemers in the area to fall off the ledge they were on.

Geemers are also seen in Metroid Prime, as three dimensional creatures resembling a turtle. They follow a very similar AI modified for three dimensional movements.

Samus Geemer
There is also a unique quirk involving Geemers in Super Metroid. In Crateria, there is a small enclosed room on the right side, close to the Wrecked Ship. It is visible on the map as a small rectangle surrounded by a much larger room. When Samus enters this room, a Samus-colored Geemer that is orange and has green on the tips of the spikes is visible above her. This Geemer follows a different pattern from normal Geemers, as it will match Samus' horizontal movements on top of the tube. This Geemer can be killed by the Wave Beam or Power Bombs, but the item dropped by it (usually energy or missiles, like any other enemy) can never be collected. Further, this particular passage can be approached from above, but when the player reaches it, the Geemer is absent. Since no programmer has specifically said why they put this Geemer here, it is likely that it is an easter egg.