Lord Lyon (1863–1887) was a British Thoroughbred racehorse that won the 1866 Epsom Derby, 2,000 Guineas Stakes and the St. Leger Stakes, becoming the third winner of the English Triple Crown. Lord Lyon raced until he was four-years old and was retired to stud in 1868. He is considered to be a marginally successful sire with his most notable progeny being the colt Minting and the filly Placida. He was euthanized in April 1887 after several years of failing health.

Lord Lyon was foaled in 1863 at Oakley Hall, the stud farm of General Mark Pearson which was located twenty miles outside of Oakham in Northamptonshire. He was sired by Stockwell, winner of the 1852 St. Leger and 2,000 Guineas Stakes and a seven-time leading sire. Lord Lyon's dam, Paradigm, was not "fashionably bred" being sired by Paragone, a "3-guinea" sire noted more for producing hunt horses than racehorses. Paradigm's dam, Ellen Horne, was purchased by Pearson for around 18 guineas for his wife to use as a hack horse. Paradigm was an unsuccessful racehorse, running only twice and placing in the Lavender Stakes as a two-year-old before she fractured pastern while running in the Bentinck Memorial at Goodwood forcing her retirement from racing. Like her dam, Paradigm was also used as a hack before she entered the stud. Paradigm was later heralded as a worthy broodmare. In 1864, she foaled Lord Lyon's full sister, Achievement, which won the 1867 St. Leger, Doncaster Stakes and Coronation Cup among many other races. Another full-sister, Chevisaunce, was the dam of the Epsom Oaks and St. Leger winning filly Jannette. Paradigm produced a total of 13 foals and also produced King at Arms, Man at Arms and Blue Mantle.

Lord Lyon was a bay horse that stood 15.3 hands high with "good bone, flat feet and very short pastern joints" and had four white socks. He was also described as "odd-shaped" and "not quite correctly balanced in all his points." According to his regular jockey Henry Custance, Lord Lyon was "a very slight whistler" until the condition was treated by firing (applying a hot rod, similar to pin firing) the back of his throat. Lord Lyon also possessed a docile and gentle temperament and was described by his trainer James Dover as "the quietest horse I have got."

Lord Lyon was leased by Richard Sutton during his racing career with Pearson retaining partial ownership. Lord Lyon's first racing trial occurred in September 1864 when he was still a yearling. According to Charles Gregory, trainer James Dover's groom, General Pearson was "a hard man with his yearlings" and wanted "to know the best or the worst of them" as soon as the horses began training. Consequently, the yearling Lord Lyon was walked by Gregory 17 miles in the rain from Oxford to Ilsley to be trained by James Dover in early autumn 1864. Lord Lyon won 17 of his 21 starts in his racing career. He was retired to stud at the end of his four-year-old season.

Lord Lyon's first trial race occurred on 10 September 1864 and was a half mile race at Ilsley against the two-year-old filly Jezebel. Lord Lyon carried 115 pounds and Jezebel carried 122 pounds. Lord Lyon was beaten by a neck in the race, a commendable feat given that Jezebel was a seasoned racehorse that had won the Bath Biennial and was carrying only seven more pounds than Lord Lyon in the race. He was again tried on 29 April 1865, where carrying 112 pounds he finished third a length and a half behind Rustic and Grisette. This trial, where he was ridden in a gallop for most of the race, seemed to upset Lord Lyon, and he is described as going "amiss" for a period of time after the running. Lord Lyon trained on favorably over the summer of 1865 and was tried a third time on 3 August in a three quarters of a mile race on Ilsley Downs where he beat his three-year-old half-sister Gardevisure by seven lengths while only breaking into a canter. General Pearson insisted on another trial run between Lord Lyon and Gardevisure to force Lord Lyon to break into a gallop. In his fourth trial two-weeks later, Lord Lyon again beat Gardevisure by a margin of three lengths while running at a canter.

Lord Lyon's first official start as a two-year-old occurred at Doncaster for the Champagne Stakes. With all the colts carrying 122 pounds, he ran a dead heat for the win with the colt Redan, who was owned by Lord St. Vincent. Lord Lyon's connections declined to take part in a deciding heat and Redan was allowed to walk over for the win. Lord Lyon's performance was so promising that commentary in Baily's Magazine of Sports & Pastimes asserted that, "if Lord Lyon had not been coughing so badly the day before, [they] should have heard of no dead heat." In September at the Newmarket meeting, the connections of the filly Mineral (later the dam of the 1876 Epsom Derby winner Kisber) forfeited 200 sovereigns to Lord Lyon after backing out of a match race. On 11 October, Lord Lyon won the 7-furlong Troy Stakes at Newmarket while carrying 122 pounds, beating the Duke of Beaufort's colt Mr. Pitt by a margin of three quarters of a length. At his last two-year-old engagement at Newmarket, Lord Lyon won the Criterion by two lengths from the colt Young Monarque while carrying 127 pounds.

In the winter of 1866-1867 Lord Lyon was one of the leading fancies for the Derby, although some observers doubted his stamina. Among the other leading contenders was his former stable companion, Rustic.

On his three-year-old debut, Lord Lyon started 4/7 favourite for the 2000 Guineas at Newmarket in a field of fifteen runners. Ridden by R Thomas (Harry Custance was injured), he won from Monarch of the Glen.