mad queen jane and the beauty of the runaway horse

A long time ago, when I was still a child, my father took me to a country which no longer exists. He took me to that country for one reason alone: there, next to a great palace which was a museum containing thousands of masterpieces of painting an sculpture, was a small building, a house, rather, which contained only a single painting. It hung on the wall of a dark room, so dark the painting was almost invisible. I had to look for a long time into the darkenss to discern its large frame, the life size figures portrayed in it, which seemed to move in a haze of fine ash. At last, I could half see the image on the canvas, and it made me fell a sort of cold sadness, a sense of desolation such as one feels when one weeps inconsolably and there is no one to hear.
A woman witn very pale features, perhaps beautiful, dressed in absolute black, stood by a long, heavily carved coffin resting on the ground, surrounded by long burning torches whose flames the wind made to waver madly. In the coffin there was a man. His dead face emerged from a black shrowd, and it was a face of angelic beauty. The black rings around the eyes of the lady standing by the coffin made it clear that she was suffering deeply because the death of that man.
That lady, my father whispered to me (it was so dark I could barely see him by my side) was mad Jane, queen of Castile. She married Phillip, called The Beautiful. He died during a campaing in Flanders. She continued fighting for his cause after his death, and took his coffin with her wherever she went, exposing the corpse all the time the battle lasted.
I was impressed by the story of the mad queen who loved her husband even long after he was dead. Later on in life I learnt that Phillip the Beautiful had been a very cruel man and an unfaithful husband, that being the reason why his wife had gone mad in the first place. Mad with a terrible jealousy.
Queen Jane had put the rage consuming her into war, and had been a great warrior. But the thing that really excited my curiosity had nothing to do with all that. It was, rather, that in that large painting, far away from the main figures, distant even from the battelfields depicted in the middle ground with miniature brush strokes and stunning accuracy, there was a pale horse, hardly visible, running, running madly, toward a distant, somber range of mountains whose summits were white with snow against the black sky.
I asked my father about that horse. And he told me that horses often run away to the valleys in the high gray hills looking for a mare to mate with. God had made them beautiful to the point that they were crazy, and had to be constantly watched or they would escape.
Why had the painter painted that horse running to the hills?
It was because the queen had requested it.
We shall never understand what she meant by it.