Be sure to bend down or even lay on the ground to get pictures of your minis...since they are so small, the angle you get when standing doesn't do them any good.
Also...choose a nice background, but don't stand the horse too close to something (ie. horse right in front of a bush)....there should be a lot of space between the background and the horse. Try to anyway...I do realize it's not always possible, but looks nice for horse standing pictures
You can use grain, wrappers or anything noisy to get the horses attention so there ears go forward. Mirrors also work great if the horse is interested. If your taking pictures of a stallion, another mare will get his attention as well.
Try different angles...like this one I took:
And here is an example of of background far away...
(Horse in the picture is my cousin's stallion, Fighter).
Good luck!! Hope they turn out great and be sure to share!!