Stubbs's equine masterpiece puts animal passion into the National

The most powerful and valuable thoroughbred in Britain will not be going into the stalls for today's Derby at Epsom. Instead, he will be found rearing up on his hind quarters in Room 34 of the National Gallery.

The name of the riderless horse with the untameable look in his eye is Whistlejacket. He was sired by Mogul, the Godolphin Arabian, out of a mare by Sweepstakes; he was owned by the Second Marquess of Rockingham, for whom he won a small fortune in prize money; and he was made immortal by George Stubbs.

Whistlejacket was painted in 1767 for 40 guineas. Worth rather more than that these days, the picture is one of the masterpieces not merely of British art but of world painting. Its arrival at the National Gallery - placed there on indefinite loan by a descendant of the aristocrat who commissioned it - constitutes one of the most thrilling additions to the public art of this country.

There was palpable excitement inside the museum as the huge work was hoisted on to the wall by two muscular but infinitely gentle picture-handlers wearing white gloves. As the great pictures always do, Whistlejacket immediately made its presence felt on the other works around it.

Hung alongside a line of elegant Grand Manner Augustan portraits - including Sir Joshua Reynolds' portrait of Lord Heathfield, Governor of Gibraltar, and Thomas Gainsborough's portrait of Mr and Mrs Hallett, otherwise known as The Morning Walk - this great brute of a work muscled in in no uncertain terms.

Its almost intimidating effect was summed up by one of the first members of the public to see it in its new setting. A 10-year-old boy bounced into the room, looked up at Stubbs' horse, stopped dead, stared for several seconds and then, with reverence, murmured: "Cor".

According to legend, Whistlejacket was originally commissioned as an equestrian portrait of George II, but the Marquess of Rockingham subsequently decided that he was insufficiently fond of the monarchy to go through with his original plan and ordered Stubbs to leave out the King.

Stubbs' spirited charger is the Whig view of history incarnate, an eloquent symbol of the British nation state as aristocrats such as Rockingham liked to idealise it - proud and free, having unseated the absolutist monarchy once and for all.

Compare Whistlejacket with the National Gallery's other, earlier great equestrian portrait, Charles I on Horseback, in Room 21 - and you have the before and after of British constitutional history in front of your very eyes. Charles's steed stands obediently still beneath his high and mighty burden, but no king will ever saddle Whistlejacket. We might still tolerate the monarchy, Stubbs' picture says - but we will never let them hold the reins of power again.

But the greatness of the picture lies not in its historical meanings, but in what Stubbs himself made of his unusual commission. His patron insisted not only that he leave out the King, but also that he leave the background unpainted. What to many other artists would have been an impossibly meagre subject was, to Stubbs, a chance to omit everything except the bare essentials.

Our attention is compelled by the bulk and daunting power of this incomparably painted animal. This lends the picture a whiff of scientific inquiry, the horse isolated as one might a specimen. But, pulling against that, there is the sheer life of the horse. Stubbs' picture is an emblematic compression of animal energy itself.

Stubbs had been to Rome in his youth and he surely intended his picture, so like a bas-relief in effect, to evoke Greco- Roman grandeur and monumentality. But the picture looks forward as well as back.

It prefigures the horse paintings of Gericault (see the National Gallery's Horse Frightened by Lightning, Room 41, which is deeply indebted to Stubbs), of Delacroix and of Picasso.

It was Stubbs who showed all those painters that to paint animals can be a way of grasping aspects of the human predicament that had seemed beyond the reach of art.

As Robert Hughes has tellingly written, the eventual progeny of the Stubbs horse would be "the horse in Guernica, thrusting its outraged neck towards the indifferent sky of the 20th century".