Which horse was the most expensive ever?

The most expensive racehorse ever was the 2000 Kentucky Derby winner, Fusaichi Pegasus, who raced just five times for trainer Neil Drysdale, but nonetheless accrued nearly £1 million in prize money and was sold to Coolmore Stud for £53.7 million at the end of his three-year-old campaign. The sale proved excellent business for his original owner, Fusao Sekiguchi, who bought him for just over £3 million as a yearling, but less so for Coolmore Stud. The son of Mr. Prospector proved a less-than-stellar acquisition and was eventually pensioned off stallion duty, as a 23-year-old, in 2020.

Interesting, John Magnier and his Coolmore Stud associates Michael Tabor and Derrick Smith also hold the record for the most expensive horse ever sold at public auction, a two-year-old son of leading US sire Forestry, from the family of Storm Cat, called The Green Monkey. At the Fasig-Tipton Calder Sale in January, 2006, The Green Monkey was knocked down to his new owners for £12 million and put into training with Todd Pletcher. He did not see a racecourse until the following September and, when he did, was placed just once in minor company before being retired from racing in February, 2008. He subsequently stood at Hartley/De Renzo Thoroughbreds in Ocala, Florida, but was equally unsuccessful as a stallion.