- an unexpected winner in a horse race, thought beforehand to have very little chance
- an almost unknown contestant regarded by few as a likely winner
- Politics a person who gets or may get the nomination unexpectedly, often by a compromise
dark horsedark horse
- One who achieves unexpected support and success as a political candidate, typically during a party's convention.
- A little-known, unexpectedly successful entrant, as in a horserace.
(plural dark horses)
Originally an allusion to an unknown horse winning a race, as used in an 1831 novel The Young Duke by Benjamin Disraeli.