Cola — Spanish for “tail” and, more informally, “ass” — comes from the Latin for the same: coda.
Coda itself has come into English in two ways. First, coda is a music term meaning… the end! The tail is the end of the animal!
More interestingly, from coda we also get the English, coward. The Latin coda became coe– in French, dropping the -d-; and an -ard is just a person, put negatively (bastard!). Thus, a coward is literally: someone who turns his tail, and runs!