dark horse

Short definition from Wiktionary

Noun

1. 1
   Someone who possesses talents or favorable characteristics that are not known or expected by others.
2. 2
   A candidate for an election who is nominated unexpectedly, without previously having been discussed or considered as a likely choice.
3. 3
   A horse whose capabilities are not known.
4. 4
   Used other than figuratively or idiomatically: see dark, horse.

Adjective

1. 1
   Alternative form of darkhorse.