If you were to sell your horse this is what a decent picture would look like. Here are some needed things:

Photographer:
-Stand far away from horse at their shoulder on your knees (zoom in to take photo). This takes away distortions.
-If you are on manual set your camera to about 200 iso and a medium aperature (typically f10 for me, depends on your camera). The ISO gets you details like fur, and the aperature assures that all of the horse will be in focus.
-Set up the horse in a place where the background is non distracting (not the best in this photo but we had no better). No chicken wire, stalls, etc.
-Set up the horse with the light shining on it. The light should be to your back. This helps with muscle definition.

Horse owner:
-Groom your horse! Shaving is nice too if possible because it helps show of the horse's muscle the best.
-Fly spray so no swishing tails.
-Set your horse up perfectly sideways to the camera.
-Have your halter on correctly/tightly (thin halters like this one complement horse's heads the best).
-Square your horse up (this horse isn't perfectly square but it was the best we could do seeing as she is a foal).
-Have your horse standing on ground that is flat.
-Have a helper to get your horse to have it's ears up and alert.

Post-Processing (if possible/needed):
-sharpen the horse
-crop out who is holding the lead rope
-get rid of extra distractions (for instance there was a telephone pole and wires, fencing and a lawn mower that I took out of this photo.
-Levels/Curves for contrast (to help with muscle definition)

Doing these will help you sell your horse because they show off his/her conformation in the best way possible.