This picture is so, so gorgeous. The detail added in the water looks so fantastic. I think the color you chose the horse to be was a really great selection because the contrast of the light coat makes the horse pop against the dark scene.
Though I am quite a fan of how dark the picture is, I think that to some it may be -too- dark. Where you've added a small bit of lightening at the bottom left, maybe adding a bit more here in there in the clouds, and having the clouds light up and highlighted in places like the lightening is behind the clouds may have lightened the picture up a bit more without making it too bright.
Overall though, I really enjoy this picture, it's so menacing and angry looking!