Don't think I'd want to try that...not sure of the load capacity of the cargo area, but horses are generally more than a thousand pounds each, and in a regular horse trailer, the weight is directly above the axles. In a toyhauler, the weight would be behind the axles, and basically "hanging" in mid-air....
Not to mention side to side sway unless you put in some kind of stall wall to restict it. Unless you REALLY love horses, what about odors permeating the living spaces? Not only do they poop at will, but they also urinate buckets.
Although I know very little about horses, my neighbors have them. I do know horses can weigh about 1500lbs and they are a high center of gravity of shifting weight.
Sounds like a recipe for disaster. Also as mentioned all the by products of a horse would stink the joint up.IMHO. (Even if they are hay burners it can still reek)
Yes absolutely on 5th wheel TH. May wantnto install a drain in the floor and rubber matt (Rural King). Most important is making sure rear weight not overloaded. If more than one horse fill tanks to full so have good tongue weight up front. Toy haulers already have vents for fuel fumes but might want to add or keep windows open. Don't know if I'd try it with number tow TH. Happy Trails