Metroids are the titular alien species of Metroid, bioweapons created by the Chozo to destroy X Parasites. Valued due to their combat applications, they are cloned and weaponized by other species across the galaxy, primarily the Space Pirates.

On their homeworld of SR388, Metroids grow past their iconic larval form through a number of more mature ones, starting with the Alpha Metroid and going through Gamma, Zeta, and Omega before becoming a Metroid Queen.