My first impression of this: WOW!! This is such a unique idea, its just stunning to look at! Its very well executed, with little improvement needed.
Actually, when I first look at this, there is nothing to improve on! But I looked a little closer, and here are my points:
The feather on the horse looks rather chunky, especially because it is right next to the tail hair. I understand feather hair is so much thicker than tail hair (grooming grey cobs have taught me that ) but it could just use a small hard brush on smudge to make it a little bit more lifelike.
The other hair is absolutely stunning, the colours and shadows and light is perfect. The strands could be just a little thinner towards the end, but thats just personal opinion, I still think its beautiful ;3
The thing that stood out to me most is probably the background. The rain is very well done, but I fear the background picture itself is perhaps a bit too dark and blurred. Especially because the sky is very light, making the rest of it seem very dark.
Thats all I can say on it really! Its a very beautiful piece, I can't wait to see your next work