The Collaborative International Dictionary of English v.0.48:

Horseman \Horse"man\, n.; pl. Horsemen.
   1. A rider on horseback; one skilled in the management of
      horses; a mounted man.
      [1913 Webster]

   2. (Mil.) A mounted soldier; a cavalryman.
      [1913 Webster]

   3. (Zool.)
      (a) A land crab of the genus Ocypoda, living on the
          coast of Brazil and the West Indies, noted for running
          very swiftly.
      (b) A West Indian fish of the genus Eques, as the
          light-horseman (Eques lanceolatus).
          [1913 Webster]

WordNet (r) 3.0 (2006):

horseman
    n 1: a man skilled in equitation [syn: horseman, equestrian,
         horseback rider]
    2: a person who breeds and cares for horses [syn: horseman,
       horse fancier]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "horseman":
   broncobuster, buckaroo, caballero, cavalier, cavalryman, chevalier,
   circus rider, cowboy, cowgirl, cowpuncher, equestrian,
   equestrienne, gaucho, horse soldier, horseback rider, horsebacker,
   horsewoman, jockey, knight, mounted policeman, postboy, postilion,
   puncher, rider, roughrider, steeplechaser, trick rider, vaquero



Easton's 1897 Bible Dictionary:

Horseman
   Heb. ba'al parash, "master of a horse." The "horsemen" mentioned
   Ex. 14:9 were "mounted men", i.e., men who rode in chariots. The
   army of Pharaoh consisted of a chariot and infantry force. We
   find that at a later period, however, the Egyptians had cavalry
   (2 Chr. 12:3). (See HORSE.)