Onager \On"a*ger\, n.; pl. L. {Onagri}, E. {Onagers}. [L. onager, onagrus, Gr. ?.] 1. (Rom.Antiq.) A military engine acting like a sling, which threw stones from a bag or wooden bucket, and was operated by machinery. --Fairholt. 2. (Zo["o]l.) A wild ass, especially the koulan.