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!