Light

While the piece itself is quiet stunning, I must admit there are some things to work on.

The lines are messy and inconsistent in places, and the shading seems to have gotten out of the lines it was supposed to be in, which rather ruins the effect of there being one lighting source. There's small pieces missing, and the muzzle looks rather square an unnatural, even considering your style.
Perhaps you could choose a darker shade of the main color you're working with, instead of grey and black? It would really clean up the shading and give more volume to the character.
As for the mane and tail, well...
The style fits very well, but it looks too segmented, unnaturally so. The lines to make it appear more hair-like along with the shading give it an inconsistent and messy look up close. There also seem to be random strands of lighting in the mane and tail...

Overall its a really great piece, just a few small flaws. Thank you for reading, and thank you for allowing critiques on your artwork. If anything in this critique was unfair or rude, I apologize, and if anything was null and void, then please ignore it.