Beavis and Butt-head

Beavis and Butt-head (B&B) originate from the animated MTV show that bears their name. Daria Morgendorffer originated from that show as well, but once she got her own spin-off, B&B never made an appearance on the latter, though they were referred to without being named in an off-hand remark made by Daria. However, in fanfiction the duo make many appearances, both in flashback and in the Dariaverse.

Daria in Beavis and Butt-head

 * “Daria on Beavis and Butt-head” (Outpost Daria)
 * “Beavis and Butt-head and Daria” by CF (Outpost Daria)

Beavis and Butt-head in Daria

 * April 2005 interview with Glenn Eichler on DVDaria, which mentions why Beavis and Butt-head were excluded from Daria episodes.

Beavis and Butt-head in Daria Fanfic

 * “Daria 2007: The Girl from Hope”: B&B are not named as such, but they are obviously the pair responsible for the shootings in Highland, Texas.