Everyone was expecting the red team to win, but the greens were the dark horse in the event.
A candidate who is nominated unexpectedly, without previously having been discussed or considered as a likely choice
If you describe someone as a dark horse, you mean that people know very little about them, although they may have recently had success or may be about to have success
someone in a race that is considered very unlikely to win (especially a political race); political candidate who is nominated unexpectedly; racehorse in respect of which little is known