One of my all time favorites Leonardo da Vinci. This one is especially near and dear to me because of my love for horses. The muscles and definition on the neck and chest make it seem so real. Almost as if the horse is prancing down the road. To think this is probably just a "doodle" in his journal. He loved horses and it showed in his art. He also made the giant clay horse that was to be cast in bronze for the equestrian monument but At the start of the Second Italian War in 1499, the invading French troops used the life-size clay model for the "Gran Cavallo" for target practice.
For further reading about Leonardo, Robin Maxwell's Signora da Vinci is an amazing book about his mother.