ˌdark 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.

[TahlilGaran] Dictionary of Contemporary English ▲

dark horse
1.
a person who does not tell other people about their ideas or skills and who surprises people by doing something that they do not expect.
I didn't know Linda had written a novel. She's a bit of a dark horse, isn't she?

2.
a person who wins a race or competition although no one expected them to.
17-year-old Karen Pickering could also be a dark horse for (= she could win) a medal in the European Championships.
(sometimes + for)

[TahlilGaran] English Idioms Dictionary ▲

dark-horse
[American]
a dark horse - a person who wins a race or competition although no one expected them to.
She's a dark-horse candidate for the position of company director.
(always before noun)

[TahlilGaran] English Idioms Dictionary ▲

dark horse
a political candidate who is little known to the general public
The woman candidate was a dark horse but she won the election easily.

[TahlilGaran] English Idioms Dictionary ▲

dark horse
n., informal A political candidate little known to the general voting public; a candidate who was not expected to run.
Every once in a while a dark horse candidate gets elected President.

[TahlilGaran] English Idioms Dictionary ▲