The phrase dark horse is an idiomatic expression that refers to a usually little-known person who unexpectedly wins or succeeds, especially in a competition of some sort.
The first known mention of the phrase is in Benjamin Disraeli's novel The Young Duke (1831). Disraeli's protagonist, the Duke of St. James, attends a horse race with a surprise finish: "A dark horse which had never been thought of, and which the careless St. James had never even observed in the list, rushed past the grandstand in sweeping triumph."
Example: Jane turned out to be a dark horse. Although she had suffered from a severe illness this year, she managed to win the race.