An ox was a large domestic mammal native to Earth.
Leonard McCoy stated that Khan Noonien Singh had the constitution of an ox. (TOS novelization: Space Seed) McCoy later stated that Eleen was as strong as an ox. (TOS novelization: Friday's Child)
During his first visit to Organia in 2267, James T. Kirk saw what looked like oxen pulling a wagon. (TOS novelization: Errand of Mercy)