“Sultan” at the Marquess of Exeter’s Stud, Burghley House Lambert Marshall (1810-1870)

The picture has something of this: cavalry, horse, mammal, people, seated, mare, farm, equestrian, racehorse, stallion, two, animal, pasture, group, equine, chestnut, livestock, agriculture, field.
Perhaps it’s a painting of a brown horse standing next to a man holding the reigns.