Whistlejacket is an oil-on-canvas painting from about 1762 by the British artist George Stubbs showing the Marquess of Rockingham's racehorse approximately at life-size, rearing up against a plain background.

[5] Stubbs depicts Whistlejacket rising to a levade, but with his head turned towards the viewer, in a pose comparable to a number of earlier monumental equestrian portraits, including examples by Rubens and Velázquez, but in these the emphasis is on the rider.

[8] To a greater degree than any earlier painter, Stubbs produced genuinely individual portraits of specific horses, paying intimate attention to details of their form.

Whistlejacket had already retired after a fairly successful racing career, but was painted in this unusual form to show "a supremely beautiful specimen of the pure-bred Arabian horse at its finest".

Even before the publication of his book, Stubbs's dedication to his subject reaped him rewards: his drawings were recognized as more accurate than the work of other equine artists and commissions from aristocratic patrons quickly followed.

[10] He was also a collector of art, commissioning several works in Italy on his Grand Tour in the late 1740s, but his great leisure interests were, typically for his class, horseracing and gambling.

In one account, The painting was supposedly intended to accompany a similarly sized equestrian portrait of George II by David Morier, but Rockingham then changed his mind.

[22] Another reason popularly given for it being "unfinished" is that Rockingham was so impressed by Whistlejacket's furious reaction when confronted by Stubbs working on the painting in his stable, that he ordered it hung without further decoration.

He was a Thoroughbred race horse foaled in 1749 at the stud of Sir William Middleton, 3rd Baronet at Belsay Castle in Northumberland,[23] and named after a contemporary cold remedy containing gin and treacle[citation needed].