I did horse photography professionaly for two years back in the late 70's and I worked with a Mamiya RB 67 - a medium format film camera that gave a 2-1/4" x 2-3/4" negative. I used a 127mm lens (which is a short telephoto lense for that camera) because it gave me the proper perspective to shoot the whole horse and worked well for head and shoulder shots, also.
The one essential ingredient in taking equine photographs that the owners would buy was how to pose the animal. The front legs have to be lined up slightly forward of the shoulders, the back legs have to be lined up about a foot behind the hips, the head has to be extended up and out from the shoulders and turned towards the camera so you can see both ears and the nose (about a forty-five degree turn of the head) and the ears HAVE to both be extended straight up.
You have to work very quickly once you get the horse posed, as they usually only hold it for a few seconds. Start with the legs. Then try to get the horse to turn its head to look at you. To get the ears straight up, have someone behind you snap their fingers and shoot immediately when the ears go up. If that doesn't work, have an assistant stand slightly behind you and to your right with a black umbrella. Have him open the umbrella slowly (with it facing the horse) and then quickly snap it completely open -THAT will get the horse's attention!
But that tecnique usually works only once on any one animal.
I photographed western halter classes in southern Illinois, Indiana, western Kentucky and central Missouri, English jumping horses and fox-hunting (million dollar horses) at Bridlespur Riding Club in St. Louis (owned by Augie Busch of Aneheisur-Busch) and the posing I have outlined was essential for all of the horses.
Good Luck!
Michael