[1] : The Collaborative International Dictionary of English v.0.48

unhorse \unhorse"\, 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

[2] : WordNet (r) 2.0

unhorse
     v : get off a horse syn: dismount, light, get off, get
         down
see also:
dismount light get off get down 

[3] : 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

Unhorse >> U Words
Unhorse, definition of term: Unhorse
unhorse_pag1.html