The Collaborative International Dictionary of English v.0.48:

buckaroo \buckaroo\ n.
   1. [fr. Sp. vaquero.]a cowboy, especially used of one who
      breaks broncos; -- used especially in California.

   Syn: vaquero.
        [WordNet 1.5]

   2. a fellow; a guy. [slang]
      [PJC]

WordNet (r) 3.0 (2006):

buckaroo
    n 1: local names for a cowboy (`vaquero' is used especially in
         southwestern and central Texas and `buckaroo' is used
         especially in California) [syn: vaquero, buckaroo,
         buckeroo]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "buckaroo":
   breaker, breeder, broncobuster, caballero, cattleman, cavalier,
   cavalryman, chevalier, circus rider, cow keeper, cowboy, cowgirl,
   cowman, cowpuncher, dairy farmer, dairyman, equerry, equestrian,
   equestrienne, farrier, gaucho, grazier, groom, horse soldier,
   horseback rider, horsebacker, horseman, horseshoer, horsewoman,
   hostler, jockey, knight, mounted policeman, postboy, postilion,
   puncher, rancher, ranchero, ranchman, rider, roughrider, sheepman,
   stableboy, stableman, steeplechaser, stock raiser, stockbreeder,
   stockkeeper, stockman, tamer, trainer, trick rider, vaquero