Okay first off, I must say the background is wonderful! The character fits right in. The added texture you put is a neat feature too, it makes the scene look like its from a story book.

As far as the horse goes, her head seems a bit small, or more likely her neck is set too short, which makes her head seem a little too small. Her mid section looks fat because of her neck and she looks a bit high withered. (of course this is all easily fixable )

Both her back and front lets look nice, the anatomy seems correct, especially the back legs, they look very nice.

The shading in her midsection is a bit strong, perhaps blend that in a bit next time? And her legs could use a bit more shading (especially the green one) Maybe added a little more shading and highlights to the mane and tail?

Also her breast muscles, horses have too big muscles in the front, I noticed that you made enough room for both of them but you only defined one, maybe add just a little shiver of shading in the middle to help define the other side. This will also help define her shoulder from her breast.

Her face is a bit cartoony but I like that Her eyes are small but that does not stop them from expressing emotions, so thats a plus and her little muzzle is adorable, I really like that about this piece, farther more you did a EXCELLENT job with her lips, you defined the dip in the middle of the lips but you didn't lose the cute adorable look, so very well done there.

So over all very good job, just a few little things I would fix next time if I was you.