The Metarex (Metalex in Japan) are an army of cyborgs and robots in anime series Sonic X. They are led by the Villain known as Dark Oak and have intentions to take control of the entire galaxy. They prove to be a tough foe against Sonic and friends, to sutch an extent to where Eggman himself is seen aiding Sonic and the others in multiple occasions, though this temporarily ends when Eggman allies with the Metarex.
Four of the Metarex, one including Dark Oak, are actually Cyborgs of another alien race that is all but extinct due to their own betrayl of their kind.
The Metarex come from Greengate, which is all but dead. Dark Oak, his four generals, and Cosmo, all all that are left of the Seedrians.
The Metarex took over multiple star systems, and annihilated the Seedrian threat that had the potential to oppose them. They killed the survivors of a Seedrian ship, Cosmo being the only survivor of the attack.
Dark oak is the leader of the metarex, and he had four generals who are also cyborg Seedrian.
Many named Metarex are encountered throughout the series.