It was I in Etruscan and Phonescian, where it was a /ts/ sound. The greeks vocalized it into our /z/ sound and made it Zeta, and that's how it it made it into Latin. (If you don't know what I mean by vocalizing it, make a "tssss" sound, and then shift immediately into a "zzzzz" sound. Your mouth doesn't change shape, you just engage your vocal cords.)