I reference a lot of photographs of horses from books and the net, and adapt the poses into these kind of proportions. References are the way to go if you want more convincing poses and movements. I also keep a good animal anatomy book handy; An Atlas of Animal Anatomy for Artists is excellent and cheap. I didn't know how to draw any animals a year ago, and I still have a long ways to go, but lots of study and practice will always yield results!