It has more to do with the shape of the ears, I think. If you look at a picture of a horse with his ears pricked, the ears are deep and not as narrow.
This is the best picture I can find ATM to describe what I'm talking about
: Horse Ears | Flickr - Photo Sharing!
See how they're more triangular that oval? For lack of a better description, if you were to stick your finger horizontally into a horse's ear, your finger would go farther back than what you have now.
I could be completely off the deep end; I'm no artist! You definitely have talent, though. I'll be interested to see the finished project.