(a) dark horse

1. a person who does not tell other people about their ideas or skills and who surprises people by doing something that they do not expect.

I didn't know Linda had written a novel. She's a bit of a dark horse, isn't she?

2. a person who wins a race or competition although no one expected them to.

17-year-old Karen Pickering could also be a dark horse for (= she could win) a medal in the European Championships.

(sometimes + for)