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]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "unsaddle":
   break, buck off, bust, cashier, deconsecrate, defrock, depose,
   deracinate, dethrone, disbar, discrown, disenthrone, dislodge,
   dismiss, dismount, displace, drum out, excommunicate, expel,
   give the gate, kick upstairs, liquidate, oust, overthrow, pension,
   pension off, purge, read out of, remove, remove from office,
   retire, root out, root up, strip of office, strip of rank,
   superannuate, suspend, throw off, unchurch, unfrock, unhorse,
   unplace, unseat, unthrone, uproot