Alpha Metroids are Metroids which have hatched from their familiar infant eggs, and grown. Alpha Metroids are the first in a series of Metroid evolution including Alpha Metroids, Gamma Metroids, Zeta Metroids and Omega Metroids. Oddly, Alpha Metroids are even weaker than previous infant form, which took five Missiles while frozen to take down.
Metroid II: Return of Samus
Metroid II: Return of Samus marked the first appearance of the Alpha Metroid. Samus mostly finds Alpha Metroids in the beginning stages of the game. The first one encountered rips its way out of its Juvenile Metroid stage. This Alpha Metroid is the first Metroid Samus faces in Return of Samus. They are defeated by five of Samus's Missiles.
In Metroid Fusion, the Galactic Federation holds a top secret program which tries to breed Metroids of all kinds. While an Alpha Metroid is seen in a stasis pod, Samus never faces any.
Metroid: Samus Returns
Alpha Metroids return in Metroid: Samus Returns, reprising their role from Metroid II: Return of Samus.