'onager' definitions:

Definition of 'onager'

noun

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]

noun

Asiatic wild ass [syn: onager, Equus hemionus]

From: GCIDE

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]