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]