Hi Brianna 🙂
You can use new or recycled materials, like stone, concrete, timber, metal. The poles for the frame should be strong, round posts such as old telegraph poles are good.
If you use metal siding, you should line the bottom four feet of the inside walls with plywood or heavy rubber so a kicking horse will not damage his legs.
The roof should be 1 to 2 feet higher at the front than the back and metal is the cheapest roofing option.
Hope it helps!