The Collaborative International Dictionary of English v.0.48:

Crupper \Crup"per\ (kr?p"p?r in U.S.; kr?p"?r in Eng.), n. [F.
   croupi?re, fr. croupe. See Croup the rump of a horse.]
   [Written also crouper.]
   1. The buttocks or rump of a horse.
      [1913 Webster]

   2. A leather loop, passing under a horse's tail, and buckled
      to the saddle to keep it from slipping forwards.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Crupper \Crup"per\, v. t.
   To fit with a crupper; to place a crupper upon; as, to
   crupper a horse.
   [1913 Webster]

WordNet (r) 3.0 (2006):

crupper
    n 1: a strap from the back of a saddle passing under the horse's
         tail; prevents saddle from slipping forward

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "crupper":
   back band, backside, backstrap, bearing rein, behind, bellyband,
   bit, blinders, blinds, breeching, bridle, butt, buttocks,
   caparison, cavesson, checkrein, cheekpiece, chinband, cinch,
   collar, croup, crownband, curb, gag swivel, girth, hackamore,
   halter, hames, hametugs, harness, haunches, headgear, headstall,
   hind end, hip straps, jaquima, jerk line, lines, martingale, nates,
   noseband, pole strap, posterior, rear, rear end, reins, ribbons,
   rump, saddle, shaft tug, side check, snaffle, surcingle, tack,
   tackle, trappings, tug, winker braces, yoke