If you are going to shoot horses, you really need to be there at dawn, and get the shoot done by 9-10am. Light will be too harsh after that. I have even found by 10am in the summer it is too late. Late afternoon isn't bad as long as you can find a shady area.

What kind of shoot are you doing? A conformation shoot? At liberty? You need to pay alot of attention to what is in your background as fences can be distracting(or plain ugly depending on what their fence is)

If you are doing a conformation shoot, you need to have someone who can stand the horse up properly.