In addition to what was mentioned above, Metroids can also mutate into stronger forms, as seen on Metroid 2
. From a Regular Metroid, that needs to be frozen then hit with five missiles to be defeated, they evolve to:
A bony growth appears on the Metroid, with small round eyes. It is actually more vulnerable than the original Metroid: It only takes five missiles, even unfrozen.
The Metroid grows several spikes forward and downwards, and becomes capable of launching electrical charges at its enemies. It takes ten missiles.
An almost humanoid creature, apparenting a cross between a dragon and a turtle: Only the transparent belly resembles the original Metroid. The Metroid becomes able to spit fireballs, and the bony shell protects him from back attacks. Even hitting it on the head or belly, it still takes 20 missiles to put them down.
A larger version of the Zeta, larger than a human, with rib-like growths protecting the belly. It can now shoot guided fireballs at its few foolish enemies, and is protected from back attacks just like the Zeta. It takes 40 missiles to put one of these behemoths down.
A monstruous, quadrupedal dinosaur-like alien
, it can easily swallow a person with its immense jaws. Its belly is its only vulnerable spot, unless a brave
attacker decides to try a lunge into its internal organs. It can resist nothing less than 150 missiles before falling.