My only criticism would be to advise you to add a bit of shading on the white markings on the horse. Since EVERYTHING else is shaded it looks a bit stark in comparison :) Other than that, looks pretty great!
Using a reference will help you with proportions and shapes :) your fairy is AMAZING, I wish I could draw wings like those, and I love your background. Especially those rocks! Horses are really hard to learn to draw, nearly as hard as people, so really studying their anatomy and proportions will help a lot.
My unreffed work isn't BAD but it's not as realistic as when I use a reference. And if you can't find a ref that fits, you can combine two. It's easier with photoshop though, because you can cut the head from one horse and slap it onto the body of another [which is what I did with this: Lucifer 2013 by =2close2dasun on deviantART ] rather than having to attempt to fit the two together in your mind.