The Evil Boys were the boys from a parallel universe seen in the Season Two episode, "Spookyfish".
Background
Due to the portal being opened at the Indian Burial Ground Pet Store, the "evil" counterparts of animals and some of the boys invade South Park from another universe.
Personalities
The boys' evil doubles have personalities that are contrary to their regular selves; Stan Marsh and Kyle Broflovski are mean and pick on Eric Cartman, while Eric is polite and generous. Kenny McCormick's evil double is said by evil Cartman to be wealthy and recently bought a new car.
Appearance
The only change to the boys' appearance are the mustache and goatee beards. The beards match their hair, except for Kyle, implying that he has brown hair instead of red.