Definitions for dark horse
Random House Webster's College Dictionary
a competitor that is relatively unknown or that wins unexpectedly.
a candidate who is unexpectedly nominated at a political convention.
Origin of dark horse:
a political candidate who is not well known but could win unexpectedly
a racehorse about which little is known
A candidate who is nominated unexpectedly, without previously having been discussed or considered as a likely choice.
An unexpected success.
Origin: Originally an allusion to an unknown horse winning a race, as used in an 1831 novel The Young Duke by Benjamin Disraeli.