Source : Webster's Revised Unabridged Dictionary (1913)

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.