The saying dark horse usually means an unexpected winner. For example, a presidential candidate who comes from behind to surprise everyone by winning the election could be considered the dark horse.
You might talk about a dark horse in politics or sports (there is usually some kind of competition involved), and the dark horse is the little-known competitor who unexpectedly wins. The idiom comes from horse racing, not surprisingly. A dark horse was one that gamblers didn't know much about and therefore weren't sure how to bet on. In the 1830's, the phrase spread to include people who were unknown before suddenly rising to prominence.