The Collaborative International Dictionary of English v.0.48:

Bucephalus \Bu*ceph"a*lus\, n. [L., fr. Gr. ?, lit., ox-headed;
   ? ox + ? head.]
   1. The celebrated war horse of Alexander the Great.
      [Webster 1913 Suppl.]

   2. Hence, any riding horse. [Jocose] --Sir W. Scott.
      [Webster 1913 Suppl.]