Pros: Your detailing looks amazing! Grounding looks good as well. Your hair looks REALLY good, its all going the same direction and thats important! Stock choices go very well together. Cons: The horse needs darker shadow on it, but just a bit darker. The horses shadow in the grass needs to be a lot darker because of the light. Overall: I would say this is one of your best pieces, with just a few tweeks it will look wonderful! You definitely have a good technique going so continue that for sure! Maybe work on the grounding a tiny tad more.
Okay, first off I always start off saying something positive and this image makes it easy to do so. Your overall idea is great! I love the lightness, it doesn't take away from the subject. Your stock fits perfectly, so kudos on that as well!
I think your placement is a little off. If you brought the stock down some to have it "settle" on the flat of the land, it would look a lot better. Your shadow is a little too light, but that's an easy fix!
Your flow of mane and tail, great work! You matched them perfectly! Your detailing is very soft but beautiful. This image is so natural, it's a picture. I love it. Great work overall!
One more thing, It seems empty, like you may need something else in the picture to balance it a bit more. Maybe some birds in the sky, this is simple, just use a bird brush!