It could be either way, and I agree, if it is photoshopped, it is an excellent job. The white hair below the horse image is not as white, and you cannot actually see the hairs on that part of the body as well as most other places in the picture. I think that it might be close to natural, and just enhanced a little to make it look more like a horse.
The way the white blends into the brown does not look right to me.
Edit to add:
I found more pictures, and it appears that this is truly how this little fellow is marked !