An ox (plural oxen) was a large domestic bovine, native to Earth, that was closely related to the cow/bull.
Flint recalled when the bubonic plague spread through Constantinople in the summer of 1334. He described how "it marched through the streets, the sewers. It left the city by ox cart, by sea, to kill half of Europe: the rats, rustling and squealing in the night as they, too, died." (TOS: "Requiem for Methuselah")
The Ox and Lamb was a 19th century inn run by Neelix in the holographic village of Fair Haven. (VOY: "Spirit Folk", "Fair Haven")
The children's saying, "olly olly oxen free", used by the Onlies of Earth Two during the 23rd century, made reference to this large bovine. (TOS: "Miri")