The Collaborative International Dictionary of English v.0.48:

Hostler \Hos"tler\, n. [OE. hosteler, osteler, innkeeper, OF.
   hostelier, F. h[^o]telier. See Hostel, and cf.
   Hospitaler, Hosteler.]
   1. An innkeeper. [Obs.] See Hosteler.
      [1913 Webster]

   2. The person who has the care of horses at an inn or stable;
      hence, any one who takes care of horses; a groom; -- so
      called because the innkeeper formerly attended to this
      duty in person.
      [1913 Webster]

   3. (Railroad) The person who takes charge of a locomotive
      when it is left by the engineer after a trip.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hostler
    n 1: someone employed in a stable to take care of the horses
         [syn: stableman, stableboy, groom, hostler,
         ostler]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "hostler":
   breaker, breeder, broncobuster, buckaroo, cattleman, cow keeper,
   cowman, dairy farmer, dairyman, equerry, farrier, grazier, groom,
   horseshoer, rancher, ranchero, ranchman, sheepman, stableboy,
   stableman, stock raiser, stockbreeder, stockkeeper, stockman,
   tamer, trainer