Ambrosius

What really stands out about the background is that there is so much depth. You've got a closeup of the grass in the front, which stretches back to those distant mountains. Shading and texture on the rocks was quite well done. The grass looks very natural, like it is slightly bending to the right from a light breeze. The colors and shading on it are great, I like the fact that you did all this grass without any green, going more for the drier tan tones.
Clouds look good, with the softer bluish insides being supported by contrasting white edges. The empty top third of the sky adds a dramatic effect.
Fantastic shading and highlighting on the horse, there is a lot of detail along the muscle structure. I like how you’ve defined the veins and folds of skin. You picked a nice pose to work with, too. The mane and tail flow well, and about the only issue I can pick out would be the slope of his hindquarters, it looks a little too low or abrupt to me. But overall I think you did a great job on this. This is my first critique, by the way.