early 13c., from Old French unicorne, from Late Latin unicornus (Vulgate), from noun use of Latin unicornis (adj.) "having one horn," from uni- "one" (from PIE root *oi-no- "one, unique") + cornus "horn" (from PIE root *ker- (1) "horn; head").
The Late Latin word translates Greek monoceros, itself rendering Hebrew re'em (Deuteronomy xxxiii.17 and elsewhere), which probably was a kind of wild ox. According to Pliny, a creature with a horse's body, deer's head, elephant's feet, lion's tail, and one black horn two cubits long projecting from its forehead. Compare German Einhorn, Welsh ungorn, Breton uncorn, Old Church Slavonic ino-rogu. Old English used anhorn as a loan-translation of Latin unicornis.