“Dark horse” began as a horse-racing term in the early 1800s. A dark horse was an animal that bettors knew little about. It had no reputation, no press, and no track history that people could study. In a field dominated by well-known favorites, the dark horse was the one that slipped under the radar. When it won, the surprise was genuine.

The phrase spread quickly beyond racing. By the mid-1800s, people were using it in politics to describe a candidate who came out of nowhere and claimed a nomination. Today it refers to any unexpected contender who suddenly outperforms the favorites.