I wouldn't be surprised to see this particular sort of case go a different way in the future. It was creatively argued that the "image" was subject to copyright - and the photograph is - so it must be tried in federal court as a copyright case. But it's also established legally that numerous photographs of the same subject can have their own copyrights. When trying to fight infringement claims when one "copies" or incorporates elements of a picture into another work (an unauthorized derivative perhaps?), the original image/photograph can be picked apart to see what elements of the original subject are subject to copyright protection to begin with.
In this case, the idea, the trees, grass, fog, the natural (but striking) occurrence of light? None of these elements are subject to copyright.
I'd expect photographers should be very wary of this case - if a view can be protected by copyright as an "image," what other views or "images" will be as well?
In any event, the OPs question goes beyond simple copyright and trademark. The "use" of the picture on products of various sorts is going to call into question if the name, identity, or likeness of the owner of the horse in question is somehow contained in the property or item (horse, whatever) in such a way that the owner's rights are infringed. Motschenbacher v. R.J. Reynolds Tobacco Co., 498 F.2d 821 (9th Cir. 1974) held that that California courts would afford legal protection to an individual's proprietary interest in his own identity - and in this case the identity was through a race car that in the ad in question was graphically altered, the number changed (as were all the cars' numbers) and some other details changed. In lay terms, it was decided that people could still tell "That's his car." In the potential use, will people, or the courts, be able to find, "That's his horse." in a way that there are infringements?
I'd expect that if the photographer, through some chance happening (I wouldn't know Seabiscuit from Dogbiscuit myself) happened to catch a well known and distinctively identifiable horse, and that horse was used commercially in a way that advanced or promoted a product, there could be a problem. Even if not for the photographer himself.