What species of life-draining creatures does Samus hunt?

The species of life-draining creatures Samus hunts is the Metroids. These floating, jellyfish-like organisms attach themselves to living beings and drain their energy, making them one of the galaxy’s most dangerous biological threats.

The Chozo originally engineered Metroids to combat the X Parasites, a shapeshifting organism that had infected their civilization. The Metroids were effective against the X, but their ability to absorb life energy eventually made them dangerous to other species as well. Space Pirates later sought to clone and weaponize them.

The creatures are not simply interchangeable with X Parasites or Space Pirates. X Parasites are a separate parasitic species, while the Chozo are the ancient race that created the Metroids. In Metroid II, the Galactic Federation hires Samus to exterminate the Metroid population on SR388, though she ultimately spares one hatchling.