Benjamin Disraeli, Edmund Gosse, Robert Arnot (1904). “The works of Benjamin Disraeli, earl of Beaconsfield: embracing novels, romances, plays, poems, biography, short stories and great speeches”
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."