Dark Horse is a term that was born in horse racing circles.
Click here for the origins of the term and for uses of it. (And watch the video - fun!)
These days it means anyone who comes up from behind or out of nowhere and takes first place, so to speak. For example, my current job position. I was called the dark horse by getting the job offer. They had already done several interviews and were pretty much at the end of people and ready to make a decision when my application came in and they decided to do one more interview. I got the job. *wink* Came in like a dark horse, ha!