From Longman Dictionary of Contemporary Englishdark horseˌdark ˈhorse noun [countable] 1 someone who is not well known, and who surprises people by winning a competition In the 1955 golf championship, dark horse Jack Fleck defeated Ben Hogan.2 British English someone who does not tell people much about themselves, but who has surprising qualities or abilities She’s a dark horse. I didn’t know she’d written a novel.
Examples from the Corpus
dark horse• Patterson, winner of the Euro 250 race at Mondello Park, could be the dark horse in these races.• Beware the ventriloquist, the dark horse, whose thrown voice juggles the truth.• And then there was the dark horse in the field. 