In the context of sports, when you say someone is a dark horse, what you mean is that very little is known about the person’s ability. Since people don’t know how well the individual plays, chances are, he may end up surprising his opponents — he may even end up winning the tournament!
*According to my favourite columnist, there are two or three dark horses in the tournament. *Sanjay is a dark horse for a medal in the 200 metres event.
The expression comes from the world of horse racing. I understand that when placing a bet on a horse, punters (people who bet on horses) take into account the animal’s lineage and its win-loss record. If this information is not available, then the bettors have no way of knowing what the strengths and weaknesses of the animal are. They are kept in the `dark’ about the capability of the animal. Such a horse is known as a `dark horse’. The expression was first used by Disraeli in his novel, `The Young Duke’.