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Black Tarquin (1945–1965) was an American-bred, British-trained racehorse and sire, best known for winning the St. Leger Stakes in 1948 and for his rivalry with Alycidon. In a career which lasted from June 1947 until July 1949, he ran fifteen times and won eight races. He was one of the leading two-year-olds of 1947, although he did not race after winning the Gimcrack Stakes in August. He was slow to reach peak fitness in 1948, and was unplaced in The Derby, but showed his best form in autumn when he beat a strong field in the St. Leger Stakes. His form continued into the following spring, but his career was effectively ended by his defeat in the Ascot Gold Cup. The victories of Black Tarquin led to a change in the way in which the Thoroughbred was defined in Britain.