The Collaborative International Dictionary of English v.0.48:

Equestrian \E*ques"tri*an\, a. [L. equester, from eques
   horseman, fr. equus horse: cf. F. ['e]questre. See Equine.]
   1. Of or pertaining to horses or horsemen, or to
      horsemanship; as, equestrian feats, or games.
      [1913 Webster]

   2. Being or riding on horseback; mounted; as, an equestrian
      statue.
      [1913 Webster]

            An equestrian lady appeared upon the plains.
                                                  --Spectator.
      [1913 Webster]

   3. Belonging to, or composed of, the ancient Roman equities
      or knights; as, the equestrian order. --Burke.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Equestrian \E*ques"tri*an\, n.
   One who rides on horseback; a horseman; a rider.
   [1913 Webster]

WordNet (r) 3.0 (2006):

equestrian
    adj 1: of or relating to or composed of knights
    2: of or relating to or featuring horseback riding
    n 1: a man skilled in equitation [syn: horseman, equestrian,
         horseback rider]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "equestrian":
   asinine, bovid, bovine, broncobuster, buckaroo, caballero, caprid,
   caprine, cavalier, cavalryman, chevalier, circus rider, cowboy,
   cowgirl, cowish, cowlike, cowpuncher, deerlike, equestrienne,
   equine, gaucho, goatish, goatlike, hircine, hoggish, hoofed,
   horse soldier, horseback rider, horsebacker, horseman, horsewoman,
   horsy, jockey, knight, mounted policeman, mulish, ovine, piggish,
   postboy, postilion, puncher, rider, roughrider, ruminant, sheepish,
   sheeplike, steeplechaser, swinish, trick rider, ungulate,
   vaquero