Something/someone you wouldn't expect to win but they could. The first defitinition on UD is correct, but the term can also be used to refer to sports teams.
Basically in reference to, lets say Ohio State in the 2010 NCAA March Madness Bracket challenge. Most people are not expecting Ohio State to win or be in the final four, but it's posibly they could pop up and win, or at the least make a finalfour appearance.
George predicted 4 teams he thought would be in the final four, but also figured that Ohio State would be the darkhorse of the 2010 Brackets if there was one.
"I think Kansas, Syracuse, Kentucky and Duke will be in the final four, and if there is a darkhorse I think it could be Ohio State"