A little-known, unexpectedly successful entrant, as in a horserace.
A political candidate who is nominated unexpectedly, or any other underdog.
"The Dark Horse Nebula or Great Dark Horse is a large dark nebula, which as seen from Earth, obscures part of the upper central bulge of the Milky Way. The Dark Horse lies in the southern constellation Ophiuchus (the Serpent Bearer), near its borders with the more famous constellations Scorpius and Sagittarius. The ability to see The Great Dark Horse with the naked eyes is an indication that the skies are very dark, i.e. not affected by (urban and industrial) light polution." (Wikipedia)