Dragonsegg, I think the problem with the horse's neck in 3D models is in how to reflect the real horse. You probably already know, a real horse's neck has the spine bone near the bottom of the neck at its base where it reaches the chest, so it hinges from what is nearly the neck's lowest point. There is a line of cartilage running up the topline of the neck from the withers to the skull, but none running along the bottom line. This cartilage gets pulled tight when the horse reaches down with its neck, producing a flattened topline and thinner-looking neck. As there is no equivalent on the bottom of the neck, you do not get the reverse effect on the bottom of the neck when the horse raises it.
(Nearly) every 3d horse model I have been able to examine places the 'hinge point' more centrally, which looks okay for neck up and neck forward positions, but generally looks bad with the head down. The main exception was, unsurprisingly, a horse skeleton. Harry is better than many models, although the texture tends to stretch a lot along the topline, which becomes obvious where you have markings such as dapples, and he doesn't show that little flat or concave section right in front of the withers. The mane would cover that, but I haven't managed to get the mane to drape forward/downward convincingly yet- anyone managed it?