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]