Sancho (1801–September 1809) was a British Thoroughbred racehorse and sire best known for winning the classic St Leger Stakes in 1804. In a racing career which lasted from May 1804 until October 1806 he won eight of his twelve competitive races. Originally trained in Yorkshire, he was undefeated in four races as a three-year-old in 1805, culminating with his victory in the St Leger at Doncaster Racecourse. In the following season he was transferred to race in the south of England where he won a series of lucrative match races against some of the leading horses of the day. His five-year-old season proved disappointing and expensive for his owner as he was injured and beaten in both of his races. Sancho was retired from racing and showed considerable promise as a sire of winners in a brief stud career.

Sancho was a bay horse owned and bred Colonel Henry Mellish, a veteran of the Peninsular War, who engaged in duelling, prize-fighting and dog-fighting. His enthusiasm for racing of any kind was such that he once trained a pig to race and gambled on which of two raindrops would be the first to reach the bottom of a window-pane. Sancho was trained by the former jockey Bartle Atkinson.

Sancho's, sire Don Quixote was a chestnut son of Eclipse bred by Mr Taylor. Apart from Sancho, his most notable offspring was the successful breeding stallion Cervantes. Sancho's dam, an unnamed mare by Highflyer, was a sister of the St Leger winner Cowslip and of the influential broodmares Maid of All Work and Rachel.

At the time of Sancho's racing career, sweepstake races, in which a number of owners contributed a relatively small sum towards the prize money, were popular, but the most valuable events were match races with two runners. In these events, each of the owners put up half the prize, with the winner taking all. If a horse failed to appear for a race its owner usually had to pay a forfeit, typically amounting to half his original stake. Sancho, for instance, won 600 guineas for winning the St Leger in 1805, but 3000 guineas for winning a match race in the following year.

Sancho made his first appearance on 16 May 1804 at Middleham Racecourse where he defeated Lord Strathmore's unnamed roan colt (the 4/6 favourite) and two others in a two-mile sweepstakes. Later on the same day he turned out again for a three-mile race in which he was matched against older horses at weight-for-age. On this occasion he started favourite and won from Lord Strathmore's four-year-old bay colt. Two weeks later at York Racecourse Sancho was brought back in distance to contest a sweepstakes for three-year-old colts and fillies over one and a half miles. He was made the 3/1 second favourite in a field of eight runners and won from Mr Cradock's bay colt.

Sancho did not compete again until 25 September, when he was one of eleven three-year-olds to contest the twenty-ninth running of the St Leger at Doncaster Racecourse. Mellish had gambled heavily on his horse, the 2/1 favourite, but was worried by the prevailing hot, dry weather, which left the course in a difficult and dangerous state ("hard as a granite road"). The conditions took their toll as three horses fell in the race and one jockey was seriously injured. Sancho took a decisive lead in the closing stages, and his rider Frank Buckle waved his whip to Mellish (conspicuous in a white satin vest) as a signal that he had the race won. At that moment, Mellish rushed from the stand to begin collecting his winnings without waiting to see Sancho cross the line in front of Sir Henry Tempest Vane's bay colt by Sir Peter.

For the 1805 season, Sancho was moved south and was campaigned at Newmarket Racecourse in spring, making his first appearance of the year in the New Claret Stakes over the two mile "Ditch-In" course on 2 May. The race brought together the three classic winners of the previous season, with Sancho starting favourite ahead of the Derby winner Hannibal and the Oaks winner Pelisse. The race produced an unexpected result, as Lord Darlington's colt Pavilion, the outsider of the four-runner field, won from Sancho, with Hannibal third and Pelisse last. At the next meeting two weeks later, Sancho was beaten by Mr Boyce's ten-year-old Bobtail in a 500 guinea match race, but Mellish was able to claim 200 guineas later the same day, "receiving forfeit" when Sancho's two rivals failed to appear for a three-mile sweepstakes.

The St Leger winner's next run was at Brighton Racecourse on 26 July, when he faced Hannibal in a match race over one mile at level weights. The Derby winner was slightly favoured in the betting, but Sancho led from the start and "won easy" to claim a prize of 1000 guineas for his owner. Six days later at Lewes, Sancho contested an even more valuable match, when he was pitted against Pavilion for a prize of 3000 guineas. Frank Buckle on Sancho tracked Sam Chifney on Pavilion for most of the four mile distance before taking the lead in the closing stages to win by half a length. "Immense sums" had reportedly been wagered on the outcome. Two days after beating Pavilion, Sancho met Bobtail, the only other horse to have defeated him, in a one-mile match at the same venue. Receiving eleven pounds from Boyce's horse, Sancho defeated his older rival to win a further 200 guineas. There was some controversy afterwards when it was discovered that the Clerk of the Course had started the race in the wrong place, meaning that the horses had run further than the agreed distance, but the result was allowed to stand.