A horse trailer would have a strong enough floor and underguts.
Shires, Belgians, Clydes, etc can weigh well over a ton and about 60% of that is on the front hooves, so figure the floors can take point loads.
I was given an 18 foot travel trailer that was junk and built a 16 foot flatbed out of it. The frame was 4 inch channel iron that was 1/4 inch thick. I bought 1/4 x 4 flat iron an boxed the frame, then added 3 inch channel cross members every three feet. I beefed up the original hitch with 2 x 4 tubing and put a 2x deck on it. I've hauled three ton of wood pellets on it with no problem and haul probably 8 loads of hay with it every year. After scrapping all the aluminum and items that worked out of it I only had to come up with the money for the decking. It is a cheap first trailer.