Okay so FINALLY I have 40min to write the decent critique you asked for.^^

Overall, we are looking at a scene that depicts the intimate moment of a living being longing to be with someone unreachable, who is, however, still there, somehow.

First, the depth within this theme is awesome. You can do so much with it and as a viewer you wonder why and how it all came to this. And, I need to say this, GREAT job on picking the stock for the living horse. This pose is so full of desire and longing you can almost feel it yourself.

On the other hand, I feel like the snow horse is kinda indifferent towards the other one. Idk, maybe you did it on purpose (to make the longing of the living horse stand out even more) but at the same time it looks like the snow horse doesn't really care about the longing of the living.
MAybe you could have tried to give it a little more emotion (difficult to find stock, I know, especially since it's "flying"), to make it look like it's trying to comfort the living horse, and sad at the same time that it can't really help its lover/friend/whatever. Maybe the noses could (almost) touch or something. You know... what horses do when they like each other.^^

In terms of originality, it is, as the vision is bigger, far more original than the last one I critiqued, but at the same time... well. You know as well that this idea has been there before (aka The Lion King) - but as this is not a horse in a pond you'll still get 4 stars for that one.^^

Technique... *switches back to the page to look at it closely* ...
... The horse - looks a little bit too smudged. I miss the winter-fur texture... You don't need to smudge it that much and it'll be fine. (Don't worry, I did that as well^^).
... The snow horse - I agree with peaches, some bigger, some smaller, some lighter and some darker snowflakes would make it more realistic and give it more depth. Plus, you could have made them a little "fuzzier", right now they are a bit too sharp in my opinion. Other than that, it is a great idea and you realised it very well. (I would have been too lazy I bet ;D).
... The snow - could use some more variety in terms of direction. When I look at the breath of the horse, the wind does not seem to be that straight, but rather, well, indefinite.
... Lighting: I miss a light source. I know, it's at night and it is cloudy (since it couldn't snow otherwise^^) but I miss a realistic or, well, fantasy light source. Like this, the light is not clearly defined, which is why you put much more light on the ground than on the horse. (This is hard, I know... one suggestion: you could make the snow glow. That'd give it some magical, mysterious feeling and light up the horse from below).
I'll still give you 4 stars for technique, because despite all my critique, this is not that easy to paint and you still did it pretty decently. The things I mentioned are kinda small compared to the thing you did RIGHT.^^

Impact, finally... well as I said, I love the pose of the real horse. It is really, really strong, it makes it easy for me to feel its desire and longing and pain. Therefore, you'll get 5 stars. Not because it'll change the world, but simply because this is one of the few horse manips here on dA that I won't forget that soon...