Thormanby (1857–1875) was a British Thoroughbred racehorse and sire. In a career that lasted from May 1859 to July 1861, he ran twenty-four times and won fourteen races. He was regarded by experts as one of the outstanding horses of his era. After winning nine races, including the Gimcrack Stakes at York as a two-year-old in 1859 he won The Derby on his first start of 1860. Although he failed to reproduce his best form in the autumn of his three-year-old season, he returned in 1861 to win his first four races including the Ascot Gold Cup.