This is really good, and I would love to order from your future (hopefully!) shop! :3 I think that she is a very cute charrie, and you have a lot of promise as an artist.
But I do have a couple critiques, although this is a lot better than I could do on the oekaki. I do spend a lot of time around horses/ studying horses, though, so I know what I'm talking about. XD
-try to thicken up and lengthen the neck up a bit
-if the horse is moving, the tail would not be in that position.

Try to give it a bit (don't over do it!) of movement, not a lot. I like how you used the smudge tool.
-The chest and stomach heights are a lot different from each other, but you can fix however you want: make the chest smaller or the stomach thicker. One will work and the other will not, I can't tell until I see it. XP
I like your style, especially the eyes and head. :3 Not realistic, yet not cartoon. A nice mix of the two. 8D