1) A bisexual bison is a member of either extant bison species which, despite some unproductive mating time, tends to contribute more to breeding programmes as it is a bovid nympho that just isn't fussy.
2) A bisexual bison can, more frequently, refer to a person who swings both ways in both appearance and sexuality, simply because they are so F%*!ing ugly that that is the only way they will ever satisfy their unhealthy sex drive. Frequently they rely on alcohol and other intoxicating substances to fool people into thinking that they're just a bit ugly, before they realise that the person is a total fucking jizz-pit
"Good morning Jenkins. By the way, the wife and I engaged in a Menage a Trois last night."
"Jolly good, Smith. Was it spiffing?"
"Unfortunately, the third was a bit of a Bisexual Bison."
"How absolutely rotten. Better luck next time, old chap!"