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.