Humble Winner. Someone who is better poised to win than obvious candidates but no can tell due to their down to earth and nonchalant mannerism. Someone who doesn't like to brag and boast or outshine others, even when superior than others.
He is the dark horse who stays cool and humble. He has quite confidence of the genius who doesn't need to flaunt his effortless brilliance.
A sexual act where one naked man sits on another mans hairy back in the riding position and jerks him off from underneath, and just before climaxing the 'horse' is forced to neigh and the 'jockey' shits all down his back, making a tail.
"Yo Steve we're doing the dark horse tonight."

"Please noo..."

"Please yes big boy"
by JohnEBlaze August 2, 2009
A pretty awesome comic book company, responsible for all the Star Wars comic books and graphic novels, among others.
You can pick up Dark Horse comics at your local comic book store.
by Escalus August 28, 2005
A song (a great one too, although not terribly well sung or produced) and the title track of the 1974 Apple album of the same name by the late, great George Harrison. Dark Horse was also the name of the visionary guitarist and composer's record label (distributed by the Brothers Warner), which he recorded seven albums for after leaving Apple (the label the Beatles formed in 1968) in 1976. Harrison himself was also known as the Dark Horse of the Beatles, a reference to a racehorse that people bet little on in the beginning, but who ultimately outshines his competitors in the end. When you have Lennon, McCartney and Martin as your mentors and Clapton, Dylan, and Shankar as three of your best friends, you can't help but radiate brilliance, as George did — and still does. As the Man himself sang:
"I'm a dark horse
Running on a dark race course
I'm a blue moon
Since I stepped out of the womb
I've been a cool jerk
Looking for the source
I'm a dark horse."
by Victor Felix September 20, 2005
Used to describe a black man with an abnormally large penis; frequently compared to the size of a horse's cock>
"YO Tanisha, I had no idea Devonte was a Dark Horse! Imean i really wasn't ready for that>"
by SessyPuertoricanPimpo September 1, 2016
Not the main favored candidate. Someone somewhat unkown who exceeds expectations of all others in an event. This saying is said to go back to the Victorian politician Benjamin Disraeli who, apart from finding time to regularly become Prime Minister, also found time to write a novel called "The Young Duke". In the book there is a description of a horse race in which the two favourites cannot make the running. In the meanwhile "a dark horse...rushed past the grandstand in a sweeping triumph".
Georgia Tech was a dark horse in the 2004 NCA basketball tournament.
by King May 7, 2005
A mysterious or shy person who unexpectedly turns out to be the best at something.

Popularly used to describe a man who may not initially catch a lady’s attention but is then unpredictably, completely charming (and, if the lady really hits the jackpot, is fantastic in bed). Due to the element of surprise, a dark horse can often beguile even the most beautiful and intelligent ladies into bed.
‘You got Lizzie into bed? Damn, you’re a dark horse man. We’ve all been trying for months’
by MJ79 March 7, 2006