Definition from Wiktionary, the free dictionary
Originally an allusion to an unknown horse winning a race, as used in an 1831 novel The Young Duke by Benjamin Disraeli: ″[...] and a dark horse, which had never been thought of, [...] rushed past the grand stand in sweeping triumph.″
- (idiomatic) Someone who possesses talents or favorable characteristics that are not known or expected by others.
- (idiomatic, politics) A candidate who is nominated unexpectedly, without previously having been discussed or considered as a likely choice.
possessor of unexpected talents or favorable characteristics