Rogues

Rogues do evil deeds alone with no henchmen, co-conspirators or other helpers. They are similar to and overlap with those such as One-Man Armies and Thrill-Seekers in that they single-handedly fight heroes. Rogue villains may be the only ones in the stories that they are in (if stories have multiple villains and/or storylines, they may instead be the only ones of the particular storylines they appear in).

This also fits for those who once worked with/for other villains or the light side, but were exiled, left to fulfill their purposes, never a part of anything (i.e. Animals and Criminals) or other reasons best explained by the pages below.