Etymology of bestiary by etymonline

c. 1200, beste, "one of the lower animals" (opposed to man), especially "a four-footed animal," also "a marvelous creature, a monster" (mermaids, werewolves, lamia, satyrs, the beast of the Apocalypse), "a brutish or stupid man," from Old French beste "animal, wild beast," figuratively "fool, idiot" (11c., Modern French bête), from Vulgar Latin *besta, from Latin bestia "beast, wild animal," which is of unknown origin.The word was used in Middle English to translate Latin animal. It displaced Old English deor (see deer) as the generic word for "wild creature," only to be ousted 16c. by animal.