a dark horse = σιγανό ποτάμι, κρυφό ταλέντο, απρόβλεπτος αντίπαλος, αουτσάιντερ

[Theseus]

http://www.phrases.org.uk/meanings/108200.html

1. (British & Australian, sometimes humorous) 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 (sometimes + for )
--17-year-old Karen Pickering could also be a dark horse for (= she could win) a medal in the European Championships. :clap:

 

[SBE]

I thought σιγανοπαπαδιά, although I think it has a slightly different meaning.

 

[nickel]

Let''s see what I have:

αουτσάιντερ
κρυφό ταλέντο
σκοτεινός αντίπαλος
αστάθμητος παράγοντας
απρόβλεπτος αντίπαλος

 

[Inachus]

2. a person who wins a race or competition although no one expected them to (sometimes + for )
--17-year-old Karen Pickering could also be a dark horse for (= she could win) a medal in the European Championships.

I think in this context would also match "Cinderella" (=a girl who achieves fame after being obscure). Σταχτοπούτα στα ελληνικά.

 

[cougr]

Perhaps ο/η κρυψίνους (ie a person who keeps their interests and thoughts/ideas secret) might be applicable in some cases.

 

[Themis†]

Σιγανό ποτάμι.