Ok, don't jump to conclusions, I am NOT finished with the Wickers portrait on the left! It is, however, slowly getting there, so I thought I'd take a break and show some anatomical landmarks from an actual equine skull, and where they are showing up on the oil painting.
The skull on the right is not Wicker's skull!! She is still alive and happily sleeping since at this moment, here in Virginia it's pretty warm & humid, so the horses are outside grazing overnight and sleeping in their stalls during the day.
You can see that the skull (right) does not line up perfectly with Wicker's head. For example, Wicker's eye is lower down on her face, whereas her infraorbital foramen is higher. Of course, different ages, sizes and breeds of horses can all influence where features turn up. In addition, there can be anatomic anomalies (see "biology") that get thrown into the mix.
When you're aiming for accuracy and believability, it helps to identify major bony landmarks to make sure you've included them and to also judge whether or not you are seeing and placing them correctly.