dark horse:
(plural: ‘dark horses‘)
a person who is not known to others very well but has extraordinary talents;
a politician who is not well-known but wins an election unexpectedly;
a little-known or unknown contestant in a competition who shows extraordinary talents and wins the competition, surprising other contestants and the audience,
a human dark horse, not known but talented
Mat is the dark horse in our office; he never shows off but comes up with an answer for every problem.
When their popular sure-to-win candidate was found guilty of corruption, the party leaders brought in a dark horse, Mr James Norton whom nobody knew until then, to contest in the elections.
Everybody, including her own trainer and jockey, was surprised to see that the horse they thought was a certain loser came out as a dark horse, winning the jackpot!
