One story was that Rockingham had intended to commission an equestrian portrait of George III; Stubbs would paint the horse while two other notable portrait and landscape painters would fill in the king and the landscape respectively.
 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.
 According to Horace Walpole, on a visit to Wentworth where he was probably shown round by the housekeeper, the painting was intended as a gift for the King, but Rockingham supposedly had not bothered to support progress of the painting after falling out of favour, and ordered it hung at Wentworth Woodhouse uncompleted instead.

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.
 Stubbs produced other paintings of horses against blank backgrounds for Rockingham, nothing in the painting indicates that it is not complete, and the detail of the shadows cast by Whistlejacket's rear legs on the ground suggest that this is how Stubbs intended the picture to be seen.
One story was that Rockingham had intended to commission an equestrian portrait of George III; Stubbs would paint the horse while two other notable portrait and landscape painters would fill in the king and the landscape respectively.  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.  According to Horace Walpole, on a visit to Wentworth where he was probably shown round by the housekeeper, the painting was intended as a gift for the King, but Rockingham supposedly had not bothered to support progress of the painting after falling out of favour, and ordered it hung at Wentworth Woodhouse uncompleted instead.
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.  Stubbs produced other paintings of horses against blank backgrounds for Rockingham, nothing in the painting indicates that it is not complete, and the detail of the shadows cast by Whistlejacket's rear legs on the ground suggest that this is how Stubbs intended the picture to be seen.
Whistlejacket