Explore the Painting history of this artwork.
Stubbs's knowledge of equine physiology was unsurpassed by any painter; he had studied anatomy at York and, from 1756, he spent 18 months in Lincolnshire where he carried out dissections and experiments on dead horses to better understand the animal's physiology. He suspended the cadavers with block and tackle to better able  sketch them in different positions. The careful notes and drawings he made during his studies were published in 1766 in The Anatomy of the Horse. 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.
Charles Watson-Wentworth, 2nd Marquess of Rockingham was a Whig politician, later to be twice Prime Minister, and exceptionally rich even by the standards of that wealthy group.  In 1762 he commissioned Stubbs to produce a series of portraits of his horses, one of which was Whistlejacket.  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.  His wife wrote of her hopes that he would restrict himself to gambling "just upon the turf, for there is always a possibility of some sort of pleasure in that; but not the smallest in other sorts".  Wentworth House, as it was then known, had been "rebuilt by his father on a huge scale" and empty walls needed filling.  Horace Walpole, on the visit in 1766 mentioned below, complained of the un-landscaped park "This lord loves nothing but horses, and the enclosures for them take place of everything".
The Wentworth archives, "though unusually comprehensive, contain no clear reference to the commission to paint Whistlejacket", though some indication of the likely price comes from a receipt by Stubbs dated 30 December 1762 for "Eighty Guineas for one Picture of a Lion and another of a Horse Large as Life", probably a different picture for Rockingham's London house.  Earlier in 1762, Stubbs had painted a second portrait of Whistlejacket, with two other unnamed stallions and a groom, Joshua or Simon Cobb.According to a story in the biography of Stubbs by his friend and fellow-painter Ozias Humphrey, when the portrait was nearly finished Whistlejacket was accidentally led in front of it by a stable boy and reacted violently, treating it as a rival stallion, and lifting the boy holding him fully off the ground in his attempts to attack the painting.  The story probably originated with Stubbs himself, but is probably too good to be true; it clearly recalls Pliny the Elder's famous story of Zeuxis and Parrhasius.When Wentworth was remodelled under a later Earl Fitzwilliam, a 40-foot square "Whistlejacket Room" was created to showcase the painting, with only single family portraits by Sir Joshua Reynolds and Sir Thomas Lawrence to keep it company.  Wentworth Woodhouse ceased to be occupied by the family after World War II, and the painting was loaned to Kenwood House in London from 1971 to 1981, the Tate Gallery 1984–85, and the National Gallery from 1996 before its purchase the next year.  It is now displayed in the centre of room 34, and is framed by doorways at the end of a long enfilade so that it can be seen through ten intervening rooms from the Sainsbury Wing, at the other end of the gallery.  It is consistently among the top ten most popular National Gallery paintings in various forms of reproduction.  The painting is in "very good condition" and was "lined, cleaned and restored a few years before its acquisition."It must have returned to Kenwood after the Tate and before the National Gallery as I remember the age of a child I used to take there who was in love with the painting. 
Also to say that there’s a full size copy in the Whistlejacket Room (Dining Room) at Wentworth Woodhouse which was visible when Wentworth Woodhouse deputised for Buckingham Palace in the film about Churchill and his audiences with the King were filmed in that room.
lined York Wentworth Woodhouse Lincolnshire equine artist Kenwood House National Gallery Thomas Lawrence Charles Watson-Wentworth, 2nd Marquess of Rockingham Prime Minister gambling Earl Fitzwilliam equine physiology Guineas horseracing Ozias Humphrey Wentworth House Grand Tour Tate Gallery Joshua Reynolds Whig stallion Pliny the Elder enfilade Zeuxis and Parrhasius Marquess of Rockingham Horace Walpole