Mistress Cora (real name Cora Anthrax) is a very old dominatrix, sometimes sporting a very southern accent. She's the famed "octogenerian dark mistress" of Highland, having minions (one called Maxine), running business, being adept at whip-based combat, and piloting the gadget-filled "Sado-Mobile".
She owns a nightclub called "Mistress Cora's", providing exotic dances and other erotic entertainment; it seems mostly aimed at the women of the town. Bizarrely, Daria Morgendorffer - in ninth grade at the time - has visited Mistress Cora's at least once. (She didn't seem interested)
In "Whipping Boys" in the Beavis and Butt-head comic, the eponymous duo were mistaken as exotic dancers by Cora. When an unnamed biker girl tried to abscond with them, Cora fought for them and later pursued her in the Sado-Mobile as they were still under contract. She ran the biker to ground and captured her with the Mobile's leather-and-lace net, but then stopped caring after the real dancers turned up... big strapping men she found she had something "special" for...
In an off-canon canon moment, she answered the letters page for #17. A girl suggested Daria needs to be less frigid and go out with one of the lads; Mistress Cora said "I have one word for Daria: leather! It works wonders with the boys!" (She said it does wonders for the waistline - "check me out!") This had a horrifying legacy in a later issue.
Mistress Cora first appeared in "Door to Door", one of the earliest Beavis and Butt-head episodes: she captured and tortured the boys in exchange for giving them pledge money. She never again appeared on the show.
She appears in #26 of the comic, where she turns out to be headmistress of a private school (and removed a misbehaving student's finger) and has been since at least 1944.