A dark horse is something or someone that is perceived to be an unlikely winner and does, in fact, succeed, usually in a competition. It should always be spelled as two words, and does not need to be set apart by quotation marks.
The term was, unsurprisingly, coined in horse racing when the winner was unknown to the gamblers, sometimes by the design of the jockeys and trainers. It first appeared in print in 1831, but by 1844 it was used in the United States for political candidates who won their nominations from seeming obscurity.
Today dark horse is used worldwide in practically every arena of competition from sports to music awards shows, and even referenced in songs.
The first favorite was never heard of, the second favorite was never seen after the distance post, all the ten-to-oners were in the race, and a dark horse which had never been thought of rushed past the grandstand in sweeping triumph. [The Young Duke]
Is the UK’s new man in Brussels a dark horse? [Telegraph]
Each year there is always that dark horse playoff candidate that no one really sees coming to make the NFL playoffs and Brandt appears to think that it’s going to be the Jacksonville Jaguars. [SB Nation]
Baby do you dare to do this? Cause I’m coming at you like a dark horse. [Katy Perry]
While there feels to be a sense of sports justice about his return to his home state, even if he had chosen a dark horse contender, the sting would have been much less with this much more subdued approach. [Toronto Star]