The Collaborative International Dictionary of English v.0.48:

Unsaddle \Un*sad"dle\, v. t. [1st pref. un- + saddle.]
   [1913 Webster]
   1. To strip of a saddle; to take the saddle from, as a horse.
      [1913 Webster]

   2. To throw from the saddle; to unhorse.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unsaddle
    v 1: remove the saddle from; "They unsaddled their mounts" [syn:
         unsaddle, offsaddle] [ant: saddle]