Okay I am finally on my computer on my lunch break, so I have been able to actually open the picture haha.
So here are my thoughts on what currently sticks out to me.
1. The eye, to me it does look slightly unnatural. Of course you can do what ever color you want but making a round eye doesnt exactly like a horses real eye.
I am going to attach a picture of my 2 recent pieces just as an example as you will be able to open it up to full detail.
I mainly would just love to see a more realistic eye, i feel like that really makes a difference :D
2. Shading and Highlights in the main and tail.
Your shaping of the hair is amazing and you absolutely have the right idea but light tends to catch on a harsher level. Again you can look at mine (the floresta one would probably be a good example), of course I too am still improving on the hair shadows etc
Basically it the shading and highlights need to be slightly more crisp.
3. Body highlight and shadows.
Again kind of similar to the hair, im not sure what your body prep process is. I myself have just started to do major shadow and highlight improvement on stock. I can see that you most likely do some as well which is amazing, i feel like it changes a piece entirely, however again light does hit thing more crisp. The main example i can give is on the last pieces face, the highlights are almost too blown out. Making the size of your brush smaller will help this, adding the highlights and then using a smudging tool (im assuming your app that you use has this as its usually fairly standard). It helps you soften the edges and keeping the center more consentrated.
These are the main things I can comment on. Im still improving on my own stuff as well even in Tier 1 so some things i mention are also things im working on for myself too haha
I hope this helps!