Your biggest problems would be the urine and processed hay. Toyhaulers use plywood veneer on the walls, and the floors are not designed for that much liquid, so things are going to be pretty rank in a short time. Also, with the cargo area where the horses would ride being so far back behind the axles, I don't think the horses would be able to take the swaying and bouncing. You can find some really nice living quarters horse trailers out there now, and just remove the slant walls to haul your toys. A friend of mine does just that, and it works well.