I love my golf cart and use it to pony horses, feed, and run all over the farm. When you pony horses, rule # 1 is just go slow at first and be sure your horses are acclimated to it. Battery carts are quiet, so most horses aren't afraid of them but if you get to scooting around and someone sets back suddenly, you can have a serious neck injury on your hands. I take two horses at a time unless I have one who won't go along with company - most will - and I start a new one alongside a more experienced horse. I tie to the back with a quick release knot and I pay attention to what's going on behind me - just like driving a car, if you are on the phone, into the ipod, etc you are distracted. I give them enough lead rope that they don't get their feet against the cart, but not so much that they can get their heads down.
All that said, as long as you are careful it's a wonderful way to exercise minis, who are otherwise pretty much limited to round pens & arenas if they don't drive. They get straight line exercise instead of being run in circles, it's less boring for them and for me.
I have a track around my hayfield, we cross the creek and have a couple of good hills to work.....at least we do that when the ground isn't cracked & dry, not lately though.
Most horses catch on quickly - I've only had one or two I gave up on because they repeatedly wanted to set back. Just pay attention to your horses and to the ground, as they won't always be able to see things coming up as soon as you will.
Jan