Okay, first off LOVE the realism here. Farther more I really like were your going. First thing I would recommend is that you smudge your shading especially around the the girth. Now I notice on the left front foot you started to add some fuzz between were hoof begins and fur ends, that looks nice, I would make that a little bit more refined and put it on all 4 feet. Now with the hooves I would try and give them some texture, kinda some faint and really smudged lines up and down and side to side so it looks more realistic (here are some refs [link] [link] [link]) I like what your doing with the veins, they look good and add to the realism of the image.
Now the ears just look weird, I now you just following your ref, but I would change that. I think you should try and make the left ear back like the right one, and the right one could be a little bigger. I would also try to cover up much of the ear with the forelock (or at least thats what I do so you can't tell) Now with the mane and tail because your going for realistic, I would really right to make them look wispy, now I'm not very good at that but I can give your a awesome tutorial [link]
I also made you a red line [link]