A dark horse means a person with hidden qualities or a person who wins, although no one expected him to win. It is used as a noun. The term originated from horse races, which later began to be used in other context.
1.Every one expected Australia to win, but Bangladesh was the dark horse of the Cricket match.
2.Mr Tom became a dark horse by winning the assembly elections.
3.We can never predict the results, a dark horse may win at the end.