The Collaborative International Dictionary of English v.0.48:

Bettor \Bet"tor\, n.
   One who bets; a better. --Addison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bettor
    n 1: someone who bets [syn: bettor, better, wagerer,
         punter]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "bettor":
   adventurer, betting ring, boneshaker, cardshark, cardsharp,
   cardsharper, compulsive gambler, crap shooter, gambler, gamester,
   hazarder, petty gambler, piker, player, plunger, punter, sharp,
   sharper, sharpie, speculator, sport, sporting man, sportsman,
   tinhorn, tipster, tout, venturer, wagerer