WordNet (r) 3.0 (2006):

cowpuncher
    n 1: a hired hand who tends cattle and performs other duties on
         horseback [syn: cowboy, cowpuncher, puncher,
         cowman, cattleman, cowpoke, cowhand, cowherd]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "cowpuncher":
   broncobuster, buckaroo, caballero, cattleman, cavalier, cavalryman,
   chevalier, circus rider, cowboy, cowgirl, cowhand, cowherd, cowman,
   drover, equestrian, equestrienne, gaucho, goatherd, gooseboy,
   gooseherd, herder, herdsman, hogherd, horse soldier,
   horse wrangler, horseback rider, horsebacker, horseherd, horseman,
   horsewoman, jockey, knight, mounted policeman, pigman, postboy,
   postilion, puncher, rider, roughrider, sheepherder, sheepman,
   shepherd, shepherdess, steeplechaser, swanherd, swineherd,
   trick rider, vaquero, waddy, wrangler