- a little known, unexpectedly successful entrant
- someone who was previously unknown, but suddenly emerges into prominence
- someone who keeps their skills and ideas secret and surprises others by doing something unexpected
- a person who wins a race or competition though no one expected them to win
- You never can tell, some dark horse many come along and win a House of Lords seat.
- Though many favoured the bigger teams to win the competition, the local team was a dark horse, and surprised everyone with their dazzling performances.
- Everyone thought that the seasoned campaigner would win the election this time, but the new candidate turned out to be a dark horse and beat him comfortably.
- While Dennis is the favourite to win the contest, our own local boy Harris could turn out to be a dark horse with his talent and skills.
- No one knew about her earlier, but suddenly she is being seen as the dark horse with such amazing display of her abilities.
- A lot of people are saying that she could be the dark horse for winning a medal in the championships.
The expression, quite evidently, originated from horse racing. A dark horse was one that was not known before the race, but performed very well. It later came to be used figuratively, in its current sense. The earliest known reference is in "The Young Duke" by Benjamin Disraeli in 1831.