The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Zool.) A wild ass of central Asia, especially the
      koulan (Equus hemionus syn. Equus hemippus syn.
      Equus onager, formely Asinus onager).
      [1913 Webster +PJC]