Isonomy 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.

The Doncaster Cup is a Group 2 flat horse race in Great Britain open to horses aged three years or older. It is run at Doncaster over a distance of 2 miles 1 furlong and 197 yards, and it is scheduled to take place each year in September.