I don't have direct access to rentals but I have some property management contacts that you could contact. One thing I also tell people to do is to google property management companies in Boise and see what they have on their websites that are going to be coming up as rentals. If you call them, they may know of something in the future that may fit your needs. I also suggest people check Craigslist out and there is a website called hotpads.com which is very helpful. It has listings from individuals as well as property management companies.
As for your horses, there are stables that you can board your horse as well as individual property owners that rent pastures for horses. This would be something that would be best discovered by word of mouth or advertisements on websites, etc.
If I can be of any other type of help, please let me know.