Cactus will be officially pulling the sled in no time (aka next week).
I like to introduce the sounds & the movements of the sled from the ground before I do it mounted from the saddle. I can build a horse's confidence by dragging it & allowing the horse to follow it. Then, I slowly let the rope out so that the sled drags beside the horse & eventually behind.
Cactus did great with this. He gave it the side-eye when I first had it dragging beside him, but it was more of a look of awareness (do I need to move out of its way?) over a spooky look.
You'll see I also ran the rope up over his back. Some horses can get surprised when the rope touches their rump, so that's something I introduce from the ground as well.
Next week's update will be posted on Friday.