WordNet (r) 3.0 (2006):

dark horse
    n 1: a political candidate who is not well known but could win
         unexpectedly
    2: a racehorse about which little is known

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "dark horse":
   also-ran, aspirant, baby kisser, bare possibility, candidate,
   defeated candidate, dud, enigma, favorite son, fighting chance,
   frontier, frontiers of knowledge, gambling chance, hardly a chance,
   hopeful, hundred-to-one shot, improbability, lame duck,
   little chance, little opportunity, long odds, long shot,
   matter of ignorance, mystery, n, off chance, office seeker,
   outside chance, political hopeful, poor bet, poor lookout,
   poor possibility, poor prospect, presidential timber, puzzle,
   remote possibility, riddle, running mate, sealed book, slim chance,
   small chance, stalking-horse, terra incognita, the incalculable,
   the strange, the unfamiliar, the unknowable, the unknown,
   unexplored ground, unexplored territory, unknown quantity,
   unlikelihood, x, z