Parasite

A Parasite or Parasite Larva (called "Parasitic larva" in the beta demo) is a hardy creature native to Tallon IV, selected for Phazon mutation by the Space Pirates for its species' resistance. Parasites do almost no damage. At the beginning of Metroid Prime they are encountered in swarms, and explode upon contact, splattering their green blood on Samus' visor. They can impede vision because of this, but little else.

Oddly, though they are repeatedly stated to be indigenous to Tallon IV, the normal variant are never found once Samus makes planetfall in Metroid Prime. Parasites also appear in Metroid Prime Pinball (where they appear on the Tallon Overworld pinball stage), albeit heavily redesigned for an unknown reason, along with the mini-game Parasite Purge.

Parasite Queens are the largest known variant, seen on Frigate Orpheon in Metroid Prime and Super Smash Bros. Brawl. These creatures were created by the Pirates with Phazon. Similar-looking corpses can be seen on Elysia's Xenoresearch, suggesting that the Pirates may have been still been trying to make use of them after the incident where they forced the evacuation and crash of the Frigate Orpheon, as well as alerted Samus Aran to their operations in the area.

Metroid Prime Pinball manual
"A new strain of parasitic creature that are dangerous in numbers"

Trivia

 * The deceased parasite specimens encountered in stasis on the Frigate Orpheon are noticeably larger than the escaped living ones. This may be due to the fact that they could be older specimens or exposed to more Phazon.
 * Interestingly, the Parasites on board the Frigate Orpheon are what allow Samus to escape the ship with her life. Parasite swarms that had infested the ventilation shafts prior to Samus' arrival cause the ventilation covers to overheat and finally blow off while Samus is evacuating. This opens a new route through the ventilation shafts that must be taken since the old route is inaccessible.
 * Parasites must have DNA that is naturally very prone to mutating. This would account for the statement in the Ice Parasite's logbook scan saying that Parasites can adapt to any environment within three generations. This is an impressive feat from an evolutionary standpoint. This possibly could have appealed to the Space Pirates as they began experimenting with Phazon on the creatures.