Dark Horse

dark horse is a little-known person or thing that emerges to prominence, especially in a competition of some sort, or a contestant that seems unlikely to succeed.

The term began as horse racing parlance for a race horse that is unknown to gamblers and thus difficult to place bets on.

The first known mention of the concept is in Benjamin D'Israeli's novel The Young Duke (1831). D'Israeli's protagonist, the Duke of St. James, attends a horse race with a surprise finish: "A dark horse which had never been thought of, and which the careless St. James had never even observed in the list, rushed past the grandstand in sweeping triumph".