Onager: the wild donkey of Asia.
From Greek όνος (donkey, ass) and άγριος (wild). Άγριος derives from ἀγρός (field).
Onager is also the name for a Roman siege weapon, a type of ballista, which threw stones from a bag or wooden bucket. Evidently, this weapon had quite a kick when fired, reminding the soldiers of the powerful kick of a donkey. Hence the name, onager.