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."
Frederick Douglass, Solomon Northup, Willie Lynch, Nat Turner, Sojourner Truth (2017). “100$ REWARD ON MY HEAD – Powerful & Unflinching Memoirs Of Former Slaves: 28 Narratives in One Volume: With Hundreds of Documented Testimonies & True Life Stories: Memoirs of Frederick Douglass, Underground Railroad, 12 Years a Slave, Incidents in Life of a Slave Girl, Narrative of Sojourner Truth...”, p.1065, e-artnow
Richard Brautigan (1989). “Richard Brautigan's Trout Fishing in America ; The Pill Versus the Springhill Mine Disaster ; And, In Watermelon Sugar”, p.73, Houghton Mifflin Harcourt