ahmg so sorry it's so late! This took me an unnecessarily long time to get through. I had to restart this so many times until I could finally finish something. Alsooooo I tried to paint wet mane & tail. Not sure how I did. it was quite a hearty challenge though xP
And this is how it's done - an excellent example of image editing. In fact it's pretty much impeccable.
I really have to commend on your sense of dynamics and composition here. Having a slightly angled background and a model is a fine way to emphasize the movement and flow; all the more it adds dynamics and therefore the image comes to life.
Another strong point here are the special effects. The way you did the rain is excellent, but what's more you added depth of field effect here. Because of that background is realistically blurry (as it would be if we were talking about weather in real world XD) and we also see that foreground rain is closer to the camera/viewer. The overpainting and highlighting is great as well: it's smooth but doesn't lose all of the texture.
Concerning color scheme here, I think you've managed just fine regardless whether other like colorful images or not. A little drop cyan to support eyecolor wouldn't hurt but it's not a major flaw, if flaw at all.
Perhaps it would have been even more gorgeous if you added some additional elements to your composition to bring difference in horse manipulations which on average contain just horse and background, but simplicity here is not a bad thing at all. It's always best to leave some air in the composition rather than cramp it with all sorts of little details.
oh no problem, I try to get to as many comments as I can! C: and I'm just starting a new piece now- I guess I can upload a walk through of it with general pointers along the way! I'll try to organize it together when I have some time