Fundamental tip that I learned from master Danilo Cartacci:
Remember that the eyes of an horse move parallely, just like (as a general rule) for human beings. That implies that if one pupil is moved on one side (e.g. the part of the orbit that is closer to the front part of the head, where the nostrils are) the other pupil will be on the OTHER side (in the example, on the part of the orbit which is closer to the back of the head, where the ears are).
This is the same as with humans: if I look to the left I will have my right pupil closer to the nose and my left pupil on the side which is closer to the ears and NOT close to the nose).
Oh my god I hope I made my point clear, rereading it sounds terribly complex but it is indeed easy!
Regards,
Alessandro