Thanks to Chris for this. He says:
mini ponies in South Park destroying some hay like a boss.
That they are. The one with the saddle is at least a little smarter than his buddy, as he's not standing directly in his food. I mean I guarantee that the non-saddled one stepped in some sort of horse poo during the day and now he's in his food source. So there's probably a reason only one of them are used for rides.