I think that you should use a reference, find tutorials, look up horse anatomy because it really helps! If you can, you could go to a stable and look at a horse. How does it look? Where does the shading go? How does it move? And that. It really helps (: Good luck!
I didn't mean to sound mean or something, I understand that (: I've been riding for 6 years but people still tell me theese things when I ask for tips and me myself hasn't read a book without theese tips standing in it (: