Here are some correspondances of similar characters between Beavis 'n Butthead and Southpark that I can think of:
Stewart - Pip
Beavis and Butthead - The two dorks who are always muttering "Yeah, they're gay. huh huh huh"
The fat chainsmoking bus driver with the curlers and hairnet who also does phone sex - Mrs. Crabtree
Mr. Vandriesen - Chickenfucker
Todd - Shelley's boyfriend in "Cat Orgy"
Cornholio - Tweak
Sterculius, the god of feces - Mr. Hankey
Any others?