I'm thinking about auctioning this off, but I want to get a gauge on what I can improve on first.

I do not think the front hoof looks large, it is just a full hoof while the rest are (from the stock image) presumably in sand.

You can better improve this imag by grounding your hooves using some slight highlighting in the dirt to make it look pushed up.

Your over all tones and lighting is really spot on.

As far as highlighting of the muscles, i think your current way of doing so is pretty spot on for the piece.

Really the only thing with this particular piece that draws my eye, is how defined and highlighted the nose portion of the face is vs. the rest of the horse. Not quite sure how you would fix this if it was natural. (Arabs tend to have their noses oiled).

If it was man made, i would suggest toning it down ever slightly to match your muscle highlighting.

I would love to commission you for a piece if you were ever so willing. :)

I'm nervous about doing commissions xD I work best when I can just go with what speaks to me. And I'm not very solid with color changes yet. I've only ever done maybe two D: