Bucephalus

Alexander the Great's favorite horse, from Greek Boukephalos, literally "Ox-head," from bous "ox" (from PIE root *gwou- "ox, bull, cow") + kephalē "head" (see cephalo-).

Men called [him] Bucephalus ... of the marke or brand of a buls head, which was imprinted vpon his shoulder. [Pliny, I.220, tr. Holland, 1601]