Through the night

Firstly, I just want to say this is a beautiful piece of art, and at first glance seems photographic in its quality.
The background colours enhance the feature of the image: the unicorn's eyes, and give them a great life-like appearance. The combination of the lighting highlights on the unicorn's coat and mane, the aurora in the sky, and the dark shade of its coat create a clear commentary about the night: its not all black; its brighter than it seems. It's an invitation to the viewer to immerse themselves in the magic of the night, the mystery of the long black of the polar regions (thanks to the aurora) and maybe even in the darkness of themselves, where a brighter, bluer light may burn.
There's a little room for improvement in my opinion, and this is purely technical advice. If this was my piece, I'd space the ears a little further apart and give them more detail, as has been given to the eyes and nostrils. One nostril is higher than another. I once had a pony who pulled this face all the time to express her great disapproval of me <img src="e.deviantart.net/emoticons/b/b…" width="15" height="15" alt="" data-embed-type="emoticon" data-embed-id="366" title=" (Big Grin)"/> So perhaps this is intentional. It does have the effect of making the lower muzzle look slightly crooked, and the far nostril doesn't seem to have a line of fog-breath to match the nearer nostril. Whether to change it or not would be a very personal decision for the artist.
And just because I'm mad on unicorns, I would make the horn bigger. The silver colouring of the horn matches the image nicely and it would be great to feature this a little more, though not to take impact from those lovely eyes.
Overall, wonderful, attractive work from an artist with great knowledge of the subject and enviable artistic ability <img src="e.deviantart.net/emoticons/s/s…" width="15" height="15" alt="" data-embed-type="emoticon" data-embed-id="391" title=" (Smile)"/>