Metroid (species)

A Metroid is an alien creature.

Biology
The eponymous in-game Metroids are large jellyfish-like creatures with quadripartite nuclei. They are capable of siphoning undetectable life energy from any life form, generally causing the death of the victim in the process. This energy can also be siphoned from the Metroid in turn, allowing it to be used as a living power source. The original Metroid establishes that exposure to beta rays would cause them to multiply very quickly

Metamorphosis
Metroid II: Return of Samus established a five-stage life cycle in which those Metroids native to SR388 go through two stages of ecdysis followed by two stages of mutation, thus maturing through five previously unknown forms: Alpha, Gamma, Zeta, Omega, and Queen.