The Collaborative International Dictionary of English v.0.48:

Saddle \Sad"dle\, n. [OE. sadel, AS. sadol; akin to D. zadel, G.
   sattel, OHG. satal, satul, Icel. s["o][eth]ull, Dan. & Sw.
   sadel; cf. Russ. siedlo; all perh. ultimately from the root
   of E. sit.]
   1. A seat for a rider, -- usually made of leather, padded to
      span comfortably a horse's back, furnished with stirrups
      for the rider's feet to rest in, and fastened in place
      with a girth; also, a seat for the rider on a bicycle or
      tricycle.
      [1913 Webster]

   2. A padded part of a harness which is worn on a horse's
      back, being fastened in place with a girth. It serves
      various purposes, as to keep the breeching in place, carry
      guides for the reins, etc.
      [1913 Webster]

   3. A piece of meat containing a part of the backbone of an
      animal with the ribs on each side; as, a saddle of mutton,
      of venison, etc.
      [1913 Webster]

   4. (Naut.) A block of wood, usually fastened to some spar,
      and shaped to receive the end of another spar.
      [1913 Webster]

   5. (Mach.) A part, as a flange, which is hollowed out to fit
      upon a convex surface and serve as a means of attachment
      or support.
      [1913 Webster]

   6. (Zool.) The clitellus of an earthworm.
      [1913 Webster]

   7. (Arch.) The threshold of a door, when a separate piece
      from the floor or landing; -- so called because it spans
      and covers the joint between two floors.
      [1913 Webster]

   8. (Phys. Geog.) A ridge connected two higher elevations; a
      low point in the crest line of a ridge; a col.
      [Webster 1913 Suppl.]

   9. (Mining) A formation of gold-bearing quartz occurring
      along the crest of an anticlinal fold, esp. in Australia.
      [Webster 1913 Suppl.]

   Saddle bar (Arch.), one the small iron bars to which the
      lead panels of a glazed window are secured. --Oxf. Gloss.

   Saddle gall (Far.), a sore or gall upon a horse's back,
      made by the saddle.

   Saddle girth, a band passing round the body of a horse to
      hold the saddle in its place.

   saddle horse, a horse suitable or trained for riding with a
      saddle.

   Saddle joint, in sheet-metal roofing, a joint formed by
      bending up the edge of a sheet and folding it downward
      over the turned-up edge of the next sheet.

   Saddle roof, (Arch.), a roof having two gables and one
      ridge; -- said of such a roof when used in places where a
      different form is more common; as, a tower surmounted by a
      saddle roof. Called also saddleback roof.

   Saddle shell (Zool.), any thin plicated bivalve shell of
      the genera Placuna and Anomia; -- so called from its
      shape. Called also saddle oyster.
      [1913 Webster]

WordNet (r) 3.0 (2006):

saddle horse
    n 1: a lightweight horse kept for riding only [syn: saddle
         horse, riding horse, mount]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "saddle horse":
   bidet, carriage horse, cart horse, cavalry horse, draft horse,
   dray horse, driving horse, fill horse, filler, gigster, hack,
   hackney, hunter, jument, lead, leader, mount, pack horse, palfrey,
   plow horse, pole horse, polo pony, post-horse, remount, rider,
   riding horse, road horse, roadster, rouncy, saddler, shaft horse,
   stalking-horse, sumpter, sumpter horse, thill horse, thiller,
   wheeler, wheelhorse, workhorse