When you really look at them side by side you will see that in some things, you are drawing what you KNOW should be there, not so much what is visible there. I always say to anyone that asks me about drawing, "draw what you SEE , not what you think you see."
His mane is actually projecting forward, and does not curl over his neck at all.
His off ear is not nearly as visible (the inside ) as the near ear. His eyelid is not so perfectly almond shaped all around, and the 'arch" of the brow ridge is not a perfect arch but rather steeper on the left side.
The nose bone and jaw bone more prominant and the eyebrow bone also more prominant, casting a darker shadow. Once you start to feel the skeletal "substance" of the horse's head, it will start to have more 3 dimensionality.
To your credit, you have done the porportions extremely well; the length of head, versus the width, the size of ears, size of nostril ; all are very good. The eyeball itself is beautifully done and looks so very glassy.
As the others said, more tonal value, and more careful looking at the shapes of the bones and direction of the hair, not operating on assumptions about mane always going over the neck.
Lastly, when you look at that photo, WHAT about THAT horse stands out? So that when you make HIS portrait, it looks like HIM? When you have decided how to answer that, then make that be the thing you really try to have incorporated in your drawing, above all else.
Last edited by tinyliny; 02-09-2012 at 08:37 PM.