The Collaborative International Dictionary of English v.0.48:

Unhorse \Un*horse"\, v. t. [1st pref. un- + horse.]
   To throw from a horse; to cause to dismount; also, to take a
   horse or horses from; as, to unhorse a rider; to unhorse a
   carriage. --Cowper.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unhorse
    v 1: alight from (a horse) [syn: unhorse, dismount, light,
         get off, get down]