Yeah, I second bsmith's comments. There is always going to be that aroma......
A horse trailer will be built to haul several thousand pounds. Horses are heavy, and they usually need a couple hundred pounds of alfalfa to along with them. The trailers are built heavy, way overkill for a dirt bike. If it is a trailer that you are just going to borrow from time to time, then fine (except for the smell) but if the trailer will be yours you will be a lot better off selling it to someone who needs a horse trailer and buying a trailer better suited to your needs.
Seriously: If all you want to do is haul a dirt bike there are plenty of cheap trailers about. A very common source of trailers are trailers intended to haul a single Jet ski/PWC. People buy one, with a trailer. Then they decide to buy a second one, and buy a double trailer to haul both. Now they have a single trailer with no purpose. No one needs a single PWC trailer.....
You can often pick up a real nice PWC trailer for next to nothing. Slap a piece of 3/4" plywood down over the frame and you have an instant flat bed trailer. You can add a set of rails if you want. It will easily haul two bikes and you can squeeze three if you try hard.
Rod