Whistlejacket was foaled in 1749. His most famous victory was in a race over four miles for 2000 guineas at York in August 1759. Stubbs's huge picture was painted in about 1762 for the 2nd Marquess of Rockingham, Whistlejacket's owner and a great patron of Stubbs.
According to some writers of the period the original intention was to commission an equestrian portrait of George III, but it is more likely that Stubbs always intended to show the horse alone rearing up against a neutral background.
From The National Gallery Podcast: Episode Seven, May 2007