There is something which bugs me a lot in the way I draw the necks of horses and other ungulates, often I feel the neck looks weird, so I'm trying to learn more on the actual horse head and neck and how it may fit nicely on a human-like torso. Colored muscles are colored just to outline their shape as they are partially hidden by others.

The most relevant notes are in the images already. I started with the basic shape of the heads of anthro horses I draw and tried to figure out if it could host a believable internal anatomy, then tweaked the shape as I studied various references to understand the head of real horses. If a larger brain is assumed then the need to accomodate it pretty much dominates the whole structure, and now I understand it leaves less and less room for sinuses, nasal cavity, and a properly working pharynx. But with such a long muzzle, and assuming that mandible muscles don't need to be huge and powerful as they are in horses, probably there is still room to accomodate most useful stuff.

This is based mostly on information from veterinary books. Anatomy for artists books show the shapes and volumes but are not detailed enough to understand what muscles actually do and what actually fills the volume. The mass of a horse's head is mostly chewing muscles under a huge nasal cavity which takes up about half the volume, as opposed to a human head which is mostly brain, and this only becomes obvious when looking at detailed cross sections of both. (Altough "An atlas of animal anatomy for artists" by Wilhelm Ellenberger is often very useful, probably it is the most accurate book of its genre.)

So for this kind of reflections the big limit is finding veterinary schemes to start from... it seems that only the anatomy of a handful domestic animals has been explored as much as human anatomy. I have to sweat blood just to find a few crude schemes of animals like cetaceans, bats, bears, very small or very large birds, reptiles, etc., let alone detailed cross sections. :-P Not even editors specializing in zoology books seem to have books with that kind of information.