Why the B in B.S.?

Why bull, specifically? Why not ox or cow, goat or pig? Why not moose or rhinoceros, who must have prodigious S? Ponder for a moment the S of a brontosaurus. Why are we so particular about bulls?

We do recognize horse in the same context, and chicken, though we never say “H.S.” or “C.S.” What is it that makes the bull variety so exceptional?

I hope RDG is proud of me for asking this question.