Tim Bowen sheds some light on the origins and definition of the phrase a dark horse.
The Macmillan English Dictionary for Advanced Learners defines the expression a dark horse thus:
1. “someone with a secret, especially a secret ability, skill or achievement, that surprises you when you finally discover it”
2. “someone who wins a race, competition, election etc. that no-one expected them to win”.
The origins of the expression are often attributed to a 19th century American horse-owner who raced a black stallion. He would arrive in a strange town and pretend that his horse was just an ordinary pack horse. Then he would enter it into a race and, to the surprise of the locals, his horse would win the race and he would pocket the prize money.
Benjamin Disraeli used the idea of a dark horse outsider winning a race in his 1831 novel The Young Duke. The expression soon began to be applied to political candidates who unexpectedly won elections. The expression is still used in this way today. Perhaps the most recent dark horse candidate was Bill Clinton, who overcame a field of more favoured candidates to win the Democratic nomination in 1992 and became President in 1993.