Captain Pocklington, who wears the uniform of the Scots Guard, retired from the third regiment in 1769, the same year that Stubbs painted this group portrait. Seated on the bench is the captain's wife, Pleasance, who is probably wearing bridal clothes. The woman standing behind Pleasance is presumably Pocklington's sister, Frances.
Stubbs' fame is based on his precise and naturalistic depictions of animals, primarily horses, even in paintings such as this that are ostensibly about human matters. Stubbs lived in a world fascinated with scientific inquiry; he himself actually performed dissections of animals to fully understand their anatomy.
Stubbs' interest in the structure and complexity of living things led him to adopt a working style in which he first painted the individual figures and then completed the background and secondary details. The subjects are arranged in a friezelike pattern against the darker, more muted shades of the massive tree and fanciful landscape. Stubbs was not invited to exhibit at the Royal Academy because he had been labeled as a horse painter, and his popularity sank even lower during the romantic era. Now in an age that looks back on pioneers such as Stubbs with fascination and respect, his stature as an artist has greatly increased.
More information on this painting can be found in the Gallery publication British Paintings of the Sixteenth through Nineteenth Centuries, which is available as a free PDF https://www.nga.gov/content/dam/ngaweb/research/publications/pdfs/british-paintings-16th-19th-centuries.pdf