There are two common uses of the idiom “dark horse” that might be the meaning when the phrase is used in modern English. Both meanings refer to a competitor and are similar, though slightly different. The phrase is commonly used, with both meanings, to describe an opponent, especially in a political race, when it is uncertain how the opponent will perform.
The first and most common meaning is to describe an opponent who is a mystery. The person is not well known, and the person's abilities, or lack of abilities, cannot be judged. In politics, this use of the term might describe a candidate who has little name recognition at the beginning of a race but who rises to prominence and wins the election.
Although mystery is at the root of the first meaning, the second use of dark horse describes someone who is known, but what is known about him or her indicates that he or she is unlikely to win. To use another common idiom, this use of the phrase dark horse is similar to the phrase underdog. In this sense, it is also sometimes used to describe a racehorse that is entered as a long shot but wins unexpectedly.
The use of the term to describe a competitor in a horse race is believed to be at the origin of the phrase. The dark horse at a racetrack was a horse that was not known. As its abilities and likeliness to win, as well as its history, were a mystery, there was no reasonable way to place odds on the dark horse. The earliest known usage of the phrase dates to 1831 with its figurative use, especially in elections, being found as early as the 1860s.
James K. Polk, the 11th president of the United States, is often referred to as the dark horse president. This is because Polk did not seem like a promising candidate and was not known to most Americans at the time of his nomination. He didn’t even get his party’s nomination until the eighth ballot was taken. The opposing party used the slogan “Who is James K. Polk?” to play up his lack of name recognition. He defeated Henry Clay and won the presidency in 1844.
As with many common sayings, the term dark horse has lent itself to use in music and literature. A 1974 album by George Harrison and a 2008 album by Nickelback are titled Dark Horse. Many films, songs, and other albums also use the saying as their titles.