1: ὄνος
(Strong's #3688 — Noun — onos — on'-os )

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: ὑποζύγιον
(Strong's #5268 — Adjective — hupozugion — hoop-od-zoog'-ee-on )

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."