Isonomy (May 1875 – April 1891) was a British Thoroughbred racehorse and sire. In a career that lasted from 1877 to 1880 he ran fourteen times and won ten races. As a three-year-old he deliberately bypassed the Classics in order that his owner could land a massive betting coup in the Cambridgeshire Handicap at Newmarket. In the following two seasons he was the dominant stayer in Britain, producing a series of performances which led many contemporary observers to regard him as one of the greatest racehorses of the 19th century. He was then retired to stud, where he became a successful stallion.