From Sonic Retro
The origins of the Metarex are mysterious, as are their motives. They have traversed the galaxy, stealing Planet Eggs and objects of power, leaving whole worlds dead or dying in their wake. Among their victims are Cosmo's homeworld, along with every member of her species except her. They also implanted Cosmo with a hidden sensor, turning her into an unwilling spy.
The Metarex are incredibly powerful, able to outmatch Sonic and his allies (unless they have power-ups). There are multiple types, and they can appear in any environment.