From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
unhorse
v 1: alight from (a horse) [syn: unhorse, dismount, light,
get off, get down]