Haag combined the formal requirements of court portraiture with a naturalistic interest in animals. In works like Frederika Sophia Wilhelmina van Pruisen, te paard, the horse is a primary subject. He used clear lighting and precise brushwork to define the musculature of animals and the textures of their coats. His compositions often place these subjects in domestic or courtly settings, as seen in his documentation of the Huwelijk van Karel Christiaan van Nassau-Weilburg met prinses Carolina. This approach allowed him to document royal ceremonies while maintaining a focus on the living creatures involved.

Modern collectors value Haag for his ability to capture the personality of animals without sentimentality. His paintings of puppies or horses feel observant and honest. These prints provide a window into eighteenth-century Dutch court life through a unique lens. The clarity of his style makes his work suitable for contemporary interiors. His depictions of the equestrian world remain popular for their technical precision.