Darkhorse is a person who everyone expects to be last in a race.
Think of it this way. The person who looks like he can never get laid defies the odds and gets any girl he can by being sly,shy and cunning. Although he's not a player, he can really charm the ladies.
Lady: Hello! How are you?
Darkhorse: Good, yea. Haha. Where're you from?
Lady: Oh. Just take me now baby.
did you see that guy! I never thought such a nerdy looking boy could get that chick! That guy is definately a darkhorse.