Wasn't that Hank Williams Junior's horse? :-)
I actually tried to track that question down. From what I can tell here's the best explanation:
Bo Ceiphus, as a nickname, originates as a nickname for Hank Williams Jr. His father affectionately called him Bocephus, a dubya-esque mispronunciation of Alexander the Great's war horse, Bucephalus. Thus, one could argue that I am named after a badass war horse.
So, bottom line, It's Bush's fault.