Portrait of a Racehorse, Possibly Disguise, the Property of the Duke of Hamilton, with Jockey Up George Garrard (1760-1826)

The picture has something of this: cavalry, seated, horse, mammal, mare, equestrian, cowman, racehorse, stallion, animal, farm, equine, livestock, saddle, chestnut, mane, grassland, pony, grass.
Perhaps it’s a painting of a man riding on the back of a brown horse on a lush green field next to a wooden fence and a wooded area with trees.