Nope, never. Way to dangerous.. When the horse puts his weight on the back of the trailer it puts it in situation in which the trailer could very easily flip. Try putting a big heavy couch or 800 lb thing in the back of the trailer (if you are really that interested) and see how the trailer moves, and then imagine a (possibly) heavier thing moving around and shifting its weigh. Smaller is usually worse because they are lighter and so the weight is more likely to move the trailer. Also, what if your horse starts stomping around? Without the truck there attached to the trailer.. Its very unsafe.
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown