Watch the head. When you've traced it, you've molded what is the forelock into your horse's head, giving it a dished luck. The eye doesn't seem proportional either.
The head looks a bit small for the body. I know you traced it, and this is how it appears in the photo, but my art teacher always told me that a picture is different to real life. Although something may make sense in a real picture, because that's how it looks, people know in their mind what a horses proportions are. So sometimes you have to tweak it in order for it not to look off