[1] : WordNet (r) 2.0

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

[2] : 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

Dark Horse >> D Words
Dark Horse, definition of term: Dark Horse
dark+horse_pag1.html

Dark >> D Words
Dark, definition of term: Dark
dark_pag1.html

Dark Ages >> D Words
Dark Ages, definition of term: Dark Ages
dark+ages_pag1.html

Dark Blue >> D Words
Dark Blue, definition of term: Dark Blue
dark+blue_pag1.html

Female Horse >> F Words
Female Horse, definition of term: Female Horse
female+horse_pag1.html

Draft Horse >> D Words
Draft Horse, definition of term: Draft Horse
draft+horse_pag1.html

Dead Horse >> D Words
Dead Horse, definition of term: Dead Horse
dead+horse_pag1.html

Crazy Horse >> C Words
Crazy Horse, definition of term: Crazy Horse
crazy+horse_pag1.html

Black Horse >> B Words
Black Horse, definition of term: Black Horse
black+horse_pag1.html