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".

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.

a person or team that is no likely to win but may do so anyway

A pretty awesome comic book company, responsible for all the Star Wars comic books and graphic novels, among others.

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:

A person who is initially very reserved in a group situation (eg a workplace) and then bursts onto the scene with unexpected alcoholic and/or womanizing behaviour.

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.