"The first favourite was never heard of, the second favourite was never seen after the distance post, all the ten-to-oners were in the rear, and 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 grand stand in sweeping triumph."

Source: The Young Duke bk. 2, ch. 5 (1831). The Oxford English Dictionary has this as its earliest citation for the term dark horse, and Disraeli is frequently considered to be the coiner. However, an earlier usage is in the Edinburgh Advertiser, 24 Sept. 1822: What is termed an outside or a dark horse always tells well for heavy betters.