Not the main favored candidate. Someone somewhat unkown who exceeds expectations of all others in an event. This saying is said to go back to the Victorian politician Benjamin Disraeli who, apart from finding time to regularly become Prime Minister, also found time to write a novel called "The Young Duke". In the book there is a description of a horse race in which the two favourites cannot make the running. In the meanwhile "a dark horse...rushed past the grandstand in a sweeping triumph".
Georgia Tech was a dark horse in the 2004 NCA basketball tournament.