Alright, first of all, I love the colors on the horse. They are very pretty ^.^
Here are some things that need improved upon: Manes&Tails: You have the basic lighting down for them, now work on the shape and flow. When I make my base layer, I use a very small hard brush, then fill it in. It helps to study and sometimes paint over other manes and tails to get a more natural look ^.^
Lighting: In this particular picture, there would be almost no light on the horse, as the trees are shadowing it. Of course, there are ways that you can change the background lighting, but they can sometimes be difficult. It is easier just to paint over the horses lighting and create your own lighting. Also in this picture, there is no need for a shadow since, as I said before, the horse is in the shadows.
Also, one last thing. When creating graphics, try to visuallize how it would look in real life. In this particular graphic, it would be better if the horse was facing the opposite direction. When it is facing to the left, it would be running into the tree.
Now, with all of this said, I really like this horse and background together I see a lot of potential here, so please keep up the good work