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]

The Collaborative International Dictionary of English v.0.48:

Koulan \Kou"lan\, n. [Native name.] (Zool.)
   A wild horse (Equus onager or Asinus onager) inhabiting
   the plains of Central Asia; -- called also gour, khur,
   and onager. [Written also kulan.]
   [1913 Webster]

   Note: It is sometimes confounded with the dziggetai, to which
         it is closely related. It is gray in winter, but
         fulvous in summer. It has a well defined, dark, dorsal
         stripe, and a short, erect mane. In size, it is
         intermediate between the horse and ass.
         [1913 Webster]

WordNet (r) 3.0 (2006):

onager
    n 1: an engine that provided medieval artillery used during
         sieges; a heavy war engine for hurling large stones and
         other missiles [syn: catapult, arbalest, arbalist,
         ballista, bricole, mangonel, onager, trebuchet,
         trebucket]
    2: Asiatic wild ass [syn: onager, Equus hemionus]