Metroids

Metroids are the titular main antagonists of the Metroid franchise, being an artificially created race of life-draining parasites.

Creation
The Metroids were genetically engineered by the Chozo, a race of bird-like creatures who adopted Samus Aran, to hunt down the X Parasites, another race of life-draining pests. However, all Metroids eventually evolved into a neutral predatory race that killed every living thing in sight.

Metroid/Zero Mission
Making their first appearance in media by being the titular antagonists of the 1986 Metroid game (as well as its remake, Zero Mission), the Metroids were discovered when the Galactic Federation began researching SR388, the planet Metroids were created. The federation then shipped out the Metroids on the space-craft  Space Research Vessel Marina, only for an army of Space Pirates to hijack the ship. Samus Aran, now a grown intergalactic bounty hunter, later wiped out the Space Pirate ranks and the stolen Metroids.