Why is the ninth month called September?
in politics, 1842, an image from horse racing, in which dark is used in its figurative sense of "unknown."
Moonraker is called a "dark horse"; that is neither his sire nor dam is known. ["Pierce Egan's Book of Sports," London, 1832]
An unexpected winner. In politics, a dark horse is a candidate for office considered unlikely to receive his or her party's nomination, but who might be nominated if party leaders cannot agree on a better candidate.
: a dark-horse candidate/ dark-horse oddsnoun phrase
A person or team, esp in sports or politics, that seems very unlikely to win but might nevertheless do so (1842+ fr horse racing)