Ripper

Rippers are small floating beetle-like creatures which move horizontally back and forth, and are best known for their near-invulnerability: a Ripper can only be killed with the Super Missile or Screw Attack.

Within the games, Rippers are commonly frozen and used as platforms to reach otherwise inaccessible areas. They have been encountered only on Zebes, though their X-parasite mimics were found on the BSL Station in Metroid Fusion.

A Galactic Federation Ripper specimen was dubbed SR-478. It is unknown whether the planet SR388, also sometimes written as the very-similar SR-388, is related to this specimen name. 

Variations
Ripper IIs are red or purple-colored Rippers which travel much faster than their rust-colored counterparts, and have exhaust flames behind them, apparantly jet-propelled. These creatures generally have more bulk than common Rippers and are more rectangular in shape. It is unknown if Ripper IIs are naturally born or if the Space Pirates have somehow modified them.

The Winged Ripper is a unique Ripper, seen only during a boss fight in Metroid: Zero Mission (where they are used as platforms). The Winged Ripper's defining characteristic are its small wings which allow it to travel in a circle, rather than the usual back-and-forth patrol.

All of the Rippers seen in Metroid Fusion are X Parasites that have either infested the bodies of normal Rippers or are simply mimicking the creatures. Rippers are one of the very few creatures who have not been changed in physiology by the parasites in any way: the Ripper X can be seen as perfect imitations.

Metroid Manual
"These strange creatures do not have brains. They simply fly around sluggishly in straight lines."

"These slow creatures don't attack but fly aimlessly in straight lines. It is wiser to run away from them than to try fighting them."

Ripper II: "These Rippers are more advanced than the Rippers who appear in the rocky zone. They can spit fire and fly at high speed. They look the same as the other Rippers but they fly a lot faster."

Super Metroid Manual
"The armor of these flying creatures is so strong that standard beams cannot beat them."

Metroid Zero Mission Manual
"For the most part, these organisms just slowly fly through the air in straight lines. They are impervious to most attacks."

Official Website
"SPECIMEN ID# SR-478 ''"Exterior armor plating makes this specimen a hearty foe. Keen vision suggests a preference for deep, cavernous areas - probably to hunt for food. DANGER LEVEL: LOW."''
 * "Armor plate"
 * "Light-sensitive retinas"
 * "Powerful legs"

Trivia

 * In Metroid Prime ' s game code there is an unused creature that closely resembles a Ripper, though the name is unknown. It appears in game concept art but does not actually make an appearance in-game. It does not have any limbs, hinting that it may have been able to fly.