A same-sex relationship (or one-night-stand) between two animals of different species. The term was coined on the fly whilst observing a male cat romance the hindquarters of a sleeping male dog.
Shawn: What the fuck are they doing?
Andrew: They're doing it! That's interbeastial faggotism! Crude!
Shawn: Where's my camera? There's gotta be a newsgroup for this kind of thing.