Cow is native and very old: Old English cu, with relatives across the whole Germanic family — Dutch koe, German Kuh, Old Norse kyr, Swedish ko — from an Indo-European root for ox or cow that turns up as far afield as Sanskrit and Greek. It is one of the words that suggest what the earliest speakers of the language actually cared about.
Its interest for spelling is what sits beside it. The animal kept its native name while the meat took a French one, beef, from Latin bovem — which is, at a great distance, the same Indo-European root. So cow and beef are ultimately cousins that took different routes into English and arrived as opposites: the Germanic one for the beast in the field, the French one for the dish on the table. The same split gave pig and pork, sheep and mutton, calf and veal.
cow (n.) "female of a bovine animal," especially the domestic ox, Middle English cu, qu, kowh , from Old English cu "cow," from Proto-Germanic *kwon (source also of Old Frisian ku , Middle Dutch coe , Dutch koe , Old High German kuo , German Kuh , Old Norse kyr , Danish, Swedish ko ), earlier *kwom , from PIE root *gwou- "ox, bull, cow." Applied to the females of various large animals from late 14c. As an insulting or degrading word for a woman, 1690s. cow (v.) "intimidate, daunt the fear or courage of," c. 1600, probably [OED] from Old Norse kuga "oppress," which is of unknown origin but perhaps has something to do with the Scandinavian forms of cow (n.) on the notion of "easily herded." Related: Cowed ; cowing . also from c. 1600