Princequillo (1940–1964) was a Thoroughbred racehorse conceived in France and born in Ireland. He is known for his performances in long-distance races and his successes as a sire.

His sire, Prince Rose, stood at the Haras de Cheffreville stud farm in France and was mated to the mare Cosquilla. When World War II broke out, the pregnant mare was shipped to Ireland, where she gave birth to Princequillo. Considering the danger from German bombing and the likelihood there would be no racing for some considerable time, Cosquilla's owners shipped her and her colt to the United States.^[1]

In July 1942, Princequillo made his American racing debut. After a few races, he was purchased by Boone Hall Stable, owned by Prince Dimitri Djordjadze of Georgia and his American-born wife, Audrey Emery. They placed him under the care of future Hall of Fame trainer Horatio Luro. Princequillo won several important races at longer distances. He broke the Saratoga Race Course record for 1¾ miles and his performances were such that he is considered to be the best long-distance runner, with the exception of Kelso, in American racing history.

Retired after his four-year-old racing season, Princequillo was purchased by Arthur B. Hancock and sent to the Hancock family's Ellerslie Stud in Albemarle County, Virginia and later to their Claiborne Farm near Paris, Kentucky. At stud, he sired 64 stakes winners and became one of the most important large-heart-producer stallions.^{[citation needed]}

Princequillo was the Leading sire in North America for 1957 and 1958 and Leading broodmare sire from 1966 through 1970 and again in 1972 and 1973. Among his daughters' progeny are Mill Reef, Fort Marcy, High Echelon,^[2] Triple Crown winner Secretariat, and Secretariat's chief rival Sham. His son Prince John was Leading broodmare sire in 1979, 1980, 1982 and 1986. Princequillo's descendants include, Secretariat, Triple Crown Winner in 1973, Triple Crown winner Seattle Slew and U.S. Horse of the Year winners A.P. Indy and Cigar, the second greatest money winner of all time. Zenyatta, John Henry, California Chrome and American Pharoah are also in his line of progeny. He garnered the nickname Mr. Fixit at stud thanks to his ability to sire foals with good conformation and soundness.^[3]

Princequillo died of a heart attack in 1964 and is buried at Claiborne Farm.