Excellent work. Very good shading and detailing, I'm especially fond of the veins and the work around the eyes.
I find the base of the mane to be slightly distracting and to be of unequal quality when compared to the rest of the drawing, but it's still quite good. It's hard to judge from the quality of the scan (or picture), but I think additional contrast in some areas, notably the bridle, would have worked out better. Try to keep the thickness of your lines constant!
Otherwise, excellent job once again!