John Milton (1824). “The Poetical Works of John Milton: With Notes of Various Authors, Principally from the Edition of Thomas Newton, Charles Dunster, and Thomas Warton, to which is Prefixed, Newton's Life of Milton”, p.425
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."