I became aware of this idea while hanging out at the BLM wild horse pens in Burns, Oregon. I realized Deigo was not the only horse that needed a hill. When I came back I incorporated mounds/hills into our own BWR paddocks.
Horses are living, breathing, and language driven beings. Given the chance, they can offer insight to what they need to be well adjusted.
Give them a chance to tell you what they want/need.