The detailing on the muzzle is superb, as is the eye. It has a soft, dappled look to it that reminds me of an old sepia print photograph or tintype.

Around the nostril, though I realize you intended a color change, I think there's too little definition. Show more wrinkling in the skin around and inside the nostril, and perhaps some more on the lips too. You've got the right idea, but the nostril area in contrast to the very detailed veins and muscles of the muzzle itself looks only half-done.

Anatomywise, it all looks good, although I'd suggest showing a little more dip where the neck meets the cheek- just a little one, for a more baroque look, like you'd see in a Mustang. Comes from Spanish blood, I believe.

The think that bugs me the most is the texture of the mane you've done so far. It has a plastic look to it in comparison to the rest. To soften it up, you may want to make your main strokes with a wide brush, and do in details only lightly, so it meshes better with the rest and doesn't detract from it.

I hope you finish this, and keep working with the style.