Synonyms for Onager (same or very similar meaning)
WordNet sense 1 (medieval artillery used during sieges; a heavy war engine for hurling large stones and other missiles):
catapult, mangonel, bricole, arbalest, arbalist, ballista, trebuchet, trebucketWordNet sense 2 (Asiatic wild ass):
Equus hemionusFrom the ODE community, based on WordNetadd/edit