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.
[3] 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.
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.
[7] 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.
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").
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.
[9][10] 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 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.
The Derby winner was slightly favoured in the betting, but Sancho led from the start and "won easy"[13] to claim a prize of 1000 guineas for his owner.
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.
[17] 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.