The Arachnus was a species of high-tier predators native to SR388. While they naturally move about in a hunched-over bipedal stance, they are capable of rolling up into a ball and moving about in that fashion instead, typically doing so when threatened. They are distinguishable by their three-toothed maws, segmented eyes, elongated neck, hardened shell covering their backs, and the tremendously elongated claws on their forearms (likely their killing implements). It is unknown exactly how they can achieve the feat, but Arachnus are capable of breathing gouts of a flame-like substance. When they feel threatened, they will spend most of the time curled up into a ball, emerging only periodically to slash with their claws or breathe their "fire".
Morph Ball technology seems to be sufficient at cracking their shell, which might be grown in plates from their spinal column (in a similar fashion to the turtles of the planet Earth), indicating why this would be deadly to their kind. While they are generally considered non-sapient, they display a sense of tremendous resourcefulness, and at least one displayed the knowledge to disguise itself as a piece of Chozo technology until a sapient being came along for it to feast upon.
Unfortunately for the Arachnus species, they are only known to have been collected by the scientists of the Biologic Space Laboratory Research Station which orbited SR388, and when the BSL was crashed into the planet below and assumed to have destroyed it in the process, the Arachnus was dubbed extinct.
- Metroid II: Return of Samus (First appearance)
- Metroid IV: Metroid Fusion