Ass - Vine's Expository Dictionary of New Testament Words
Ass[ 1,,G3688, onos ]
is the usual word. Onarion, the diminutive of onos, a young ass, or ass's colt," is used in John 12:14, together with onos.
[ 2,,G5268, hupozugion ]
lit., "under a yoke" (hupo, "under," zugos, "a yoke"), is used as an alternative description of the same animal, in Matthew 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, John 12:14. In 2 Peter 2:16, it is used of Balaam's "ass."