The Countess of Coningsby turns out to be Margaret Newton, Countess of Coningsby in her own right. Her article in thepeerage is here.
From the Google Art Project via Wikimedia.
Military influence is visible in this equestrian portrait of the Countess of Coningsby, but she is wearing a wide-brimmed hat instead of a tricorne.
Keywords: 1760, Stubbs, Magaret Coningsby, Margaret Newton, Countess Coningsby, Countess, British, straight coiffure, hat, high enclosing neckline, neck band, bows, shemise, jacket, lapels, long close sleeves, cuffs, buttons, peplum, vest, full skirt, equestrian dress