Ridley

Ridley is a dragon-like boss that appears in most Metroid games. He is a leader of the Space Pirates, and Samus Aran's arch-nemesis. Throughout the games, Ridley has taken many different forms. There's normal Ridley, Meta-Ridley from Metroid Prime, "Beta"-Ridley from Metroid Zero Mission and X-Ridley from Metroid Fusion.

Meta-Ridley
After having his body severely ruined by Samus Aran in the original Metroid game, Ridley was rebuilt with metallic parts and stronger than ever. It is unclear how he goes from normal Ridley to Meta Ridley and then normal Ridley again. Meta-Ridley debuted in Metroid Prime and then appears in Metroid Prime 3 early in the game.

Mecha Ridley
It´s Ridley with a cyborg body. The Space Pirates built him with only the half of it´s body. May be this is a prototype of the following version of the resurrected Ridley: Meta Ridley.

Omega Ridley
Omega Ridley is Ridley corrupted with Phazon. He serves as the Pirate Homeworld Leviathan guardian that you must defeat in order to get the Hyper Grapple and to destroy the Leviathan.

Ridley X
The X Parasite have the ability to absorb the form & function of other living beings. When the Galactic Federation froze Ridley for imprisonment, an X Parasite managed to get to his frozen corpse, absorb his DNA, and leave. This left the original Ridley to crumble into snowflakes. X-Ridley later appears in full form. Like a clone, this Ridley is a bigger, badder, louder form of Ridley that seems to be extremely weak to a fully charged energy blast. When he picks you up, shoot your Super Missiles until he drops you. Finish off the Core X to get the Screw Attack.