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]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "unhorse":
   alight, buck off, climb down, depose, deracinate, dislodge,
   dismount, get down, get off, land, light, overset, overturn, perch,
   pitch, root out, root up, settle, throw off, topple, touch down,
   tumble, unplace, unsaddle, unseat, uproot