I have seen them at the Midwest Horse Fair. I have not used one for a mini. I know of a Cadillac cart that was purchased for a big horse. It was built like a tank. That can be good or bad. It was
VERY heavy in the shafts and the only way to balance it was to have a person stand on the back end of it. (There was no step for that, you just had to get on and hold on!)
The person that had it also had a saddle that was too narrow on the horse, and so all that weight rested on two spots the size of quarters on either side of the horse's withers. It was not a good scenario. We helped her fix all that.
If the mini carts are built the same way, I would be very skeptical of putting one on my horse. I also think the trailer thing is a bad idea. No mini should have to pull around that much weight, inc. humans. And you know my opinion about putting a pair to a cart...
Myrna