Metarex

The Metarex (Metalex in Japan) are a species of cyborgs and robots from the planet Green Gate. They are led by the villainous entity known as Dark Oak and have plan to take over the entire galaxy. They prove to be a tough foe against Sonic and friends, to such 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 betrayal of their kind.

History
The Metarex come from Green Gate, 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.

Important Metarex

 * Dark Oak: the leader of the metarex, and he had four generals who are also cyborg Seedrians.
 * Four generals:
 * Black Narcissus.
 * Pale Bay Leaf.
 * Yellow Zelkova.
 * Red Pine.