This reminds me of a lady who I gave a bascially untrained Belgian 2 yr old stud to... He had learned halter and lead work but was pretty studdy and I was going to have him fixed, but something happened and I had to move him out. Anyway, this lady lived way the heck back in the bush. No way, in any way, was a horse trailer going back there. So she told me they were going to tie the stud to the back of their quad and bring him in that way. I immediately told her it was a terrible idea. If this guy spooked he would flip the quad and who knows who would end up hurt or worse and being waaaaay back in the bush, no one would know for hours or days. After a discussion, she agreed that it would be better to hold his lead for the walk/ride in to the "farm."
I can see doing this with a full size pickup, with quick releases and supervision, but never with a quad. Even if the horses are trained. 98% of the time maybe all is OK, but it's that 2% that causes the trouble.