Eclipse (1 April 1764 – 26 February 1789) was an undefeated 18th-century British Thoroughbred racehorse who won 18 races, including 11 King's Plates.

After retiring from racing, he became a very successful sire, whose offspring included three Epsom Derby winners: Young Eclipse, Saltram and Serjeant.

It was at Cranborne that his sire, the Jockey Club Plate winner Marske (by Squirt from The Ruby Mare) stood.

[4] Instead he was turned over to a rough-rider, who worked him hard all day, and at night as well on poaching expeditions if the stories are to be believed.

[8][9] Supposedly, at this time O'Kelly used the famous phrase "Eclipse first and the rest nowhere", before making his bets for this race, although some sources[6] have him saying this for the second heat of the horse's debut.

[7] In August, he took on top class horses Tortoise and Bellario in the Great Subscription Purse at York, but at odds of 20/1 on, routed both of them, being over a furlong in front after two miles, and winning in a canter.

[6] Eclipse won 18 races, including 11 King's Plates, supposedly without ever being fully extended and proving far superior to all competition.

[5] In 1771, Eclipse was retired to stud after a racing career of about 17 months due to lack of competition as nobody was betting on rival horses.

[4][22] Eclipse was never the leading sire in Great Britain and Ireland, although he finished in second place 11 times, usually behind Herod.

A book published in 1970 stated that the Royal Veterinary College had determined that nearly 80% of Thoroughbred racehorses had Eclipse in their pedigree.

"[3] This modern dominance comes mainly through a sire line established by Potoooooooo, although King Fergus would also found another successful branch leading to Ribot.

His skeleton is now housed at the Royal Veterinary College, Hertfordshire, in the Learning Resource Centre named after him, although it cannot be said for certain whether all the bones displayed are really from Eclipse.

[153] A necropsy on Eclipse by Charles Vial de Sainbel[154] found that he had an abnormally large heart, weighing 14 pounds (6.4 kg).

[155] While the theory has yet to be proven, it has gained popularity due to the large hearts seen occasionally in his descendants, including Secretariat and Phar Lap.

The Eclipse Stakes is a Group 1 flat race in the United Kingdom for three-year-olds and older run over a distance of 1¼ miles and 7 yards (2,018 metres) at Sandown Park.

Eclipse Press is the book-publishing division of Blood-Horse Publications, an international publishing house for top Thoroughbred and general equine magazines, books, videos, CD-ROMs and annual references.

[157] At the time of Eclipse's birth, stud records were maintained by the individual breeder with no central authority.

In 1791, James Weatherby and William Sydney Towers pieced together the General Stud Book, which is the source of the pedigree shown below.

[158] According to all official records, Eclipse's sire was Marske (written as 'Mask' in a 1764 entry in the Royal Stud Book).

Spilletta's dam was Mother Western, whose paternity is subject to similar confusion between Scarborough Colt and Easby Snake.