Everyone, please see the "breaking rig" designed and used by a friend of FrogInAblender. I've never seen anything like it.
These are video clips... if you are on a slow connection, you will be better off to download them to your own computer rather than trying to stream them.
To download it, right click it and select "Save target as..." and put it on your desktop. It may take a minute to many minutes to download depending on your connection, but once it's there it will play normally!
It's worth a look!
Even the most determined horse would be hard put to rear up, run, or lay down in that (of course, getting him IN to it in the first place might be more fun than we want).
I am not a harness horse person - I have only harnessed a pony and driven him ONCE - and somebody who knew what they were doing was helping me!
She reminded me of your Pony, but she wasn't nearly as pretty. Apparently jumpiness and suspicion is an inborn trait of Hackneys coz this little girl sure was. He was given both of them by an old gentleman who had been raising Hackneys for years and was in failing health. The old man said that if they weren't "wired" they weren't any good in the show ring. So I'll bet Pony would be an excellent show harness prospect if you were ever so inclined.