I tried to pony Blade ONE time off of Tennessee and that was enough to cure me of ever wanting to do it again. Tennessee was fine with it, but Blade kept on wanting to climb up there with me, or to bite on Tennessee, or pull my arm out of it's socket, or get on the wrong side. It was a comedy of errors let me tell ya! I was riding in my normal english type saddle so I had no horn to begin with, but I wouldn't have tied to it even if I had. Too many bad things can happen.
When I was ponying Cadence with her momma, the paint mare you've seen pictures of, when she got strong I would tie a big lead rope loose around the mare's neck that would hang right at the shoulder. I wouldn't tie the foal to that, but I would run the foal's line through it, so that the foal wasn't pulling me off if she was lagging back. If I needed to, I could still just let go.
I started ponying Dot when she was 2months. But she was halter broke, would lead very well, and I did it in our arena. Belle was OK with it, so I did dally to the horn, but did not tie it. If I'd have had a problem it would have slipped off when I let go.
This girl did NOT start in an arena. I guess they went thru creeks and over rocks and across a road, where people in cars stopped to try and help. before she got the mare stopped. It sounds like it was truly as bad as it could get.
Becky