It depends on what engine you are installing the feature into and how the data handlers are aligned. In it's simplest form you can just make the horse a holdable stat change item for the attributes but it is crude, it is the imaginative element that matters here. Now if you make a slightly different Python engine for instance it is easy as hell to add a life bar for the horse along with, well, all kinds of things really. Free form as it is just data and light amounts of handling methods. You would just have to run the derived stat calculation after equip, hit eva etc. I got it working once.