Ass

is the usual word. Onarion, the diminutive of onos, "a young ass, or ass's colt," is used in Joh 12:14, together with onos.

lit., "under a yoke" (hupo, "under," zugos, "a yoke"), is used as an alternative description of the same animal, in Mat 21:5, where both words are found together, "Behold, thy king cometh unto thee, meek and riding upon an ass (onos), and upon a colt the foal of an ass (hupozugion)." It was upon the colt that the Lord sat, Joh 12:14. In 2Pe 2:16, it is used of Balaam's "ass."