Thomas Norton was the son of John Bysshe Norton (1724–1804).
A preparatory drawing for this composition in pen, ink and watercolour, is at the Yale Centre for British Art (inv. no. B2001.2.900; fig. 1).1 Differences between the drawing and the painting include the elimination of a dog directly under and behind the horse's head, thus simplifying the composition, and the alteration of the gesture of Norton's left hand. There are also differences in the placement of Norton's feet and the notable addition of the large tree-trunk with foliage in the upper left corner of the painting.
1 https://collects
ions.britishart.yale.edu/catalog/tms:46981