Other: Dark Horse

A dark horse candidate is one who is nominated unexpectedly, without previously having been discussed or considered as a likely choice. Often a dark horse is selected as a compromise when other, more prominent candidates' factions cannot come to an agreement. more...

This metaphoric expression originally alluded to an unknown horse winning a race and was so used in a novel by Benjamin Disraeli (The Young Duke, 1831).

The expression was soon applied to political candidates, among the first of whom was James Knox Polk. He won the 1844 Democratic presidential nomination over Martin Van Buren on the eighth ballot and went on to win the election. Other famous dark horse candidates for the US Presidency were Franklin Pierce, who was chosen as the Democratic nominee and later elected president in 1852, James A. Garfield, elected the 20th president in 1880, Warren G. Harding, elected president after his surprise nomination, John W. Davis, the unsuccessful Democratic nominee in 1924, and Wendell Willkie, the unsuccessful Republican nominee in 1940. More recently, some have described Bill Clinton as a dark horse, after he overcame a crowded field of Democratic hopefuls to win the 1992 presidential election.

In more recent times, the term "Dark Horse" has come to be applied to contests that work on seeding. At times, a certain person/thing/character/etc. that has a high seed (in other words, is not likely to win the contest) may begin to defy all expectations and defeat others with lower seeds, not just once, but several times in a row.

Read more at Wikipedia.org