|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.
dark horse definition