ˌdark ˈhorse BrE AmE 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.