|1.||a competitor in a race or contest about whom little is known; an unknown|
|2.||a person who reveals little about himself or his activities, esp one who has unexpected talents or abilities|
|3.||(US) politics a candidate who is unexpectedly nominated or elected|
An unexpected winner. In politics, a dark horse is a candidate for office considered unlikely to receive his or her party's nomination, but who might be nominated if party leaders cannot agree on a better candidate.
A little known, unexpectedly successful entrant, as in You never can tellsome dark horse may come along and win a Senate seat. This metaphoric expression originally alluded to an unknown horse winning a race and was so used in a novel by Benjamin Disraeli (The Young Duke, 1831). It soon began to be transferred to political candidates, among the first of whom was James K. Polk. He won the 1844 Democratic Presidential nomination on the eighth ballot and went on to win the election.