Shari, thanks for the AWESOME link!!
I've wanted a nice pannier set for years and found a few online but didn't know if they'd fit the minis or not and they were "seriously priced" for serious packers. The ones your company has look great!
To the OP, since you have a history of backpacking with horses I'm sure you're aware of how important it is that the load be comfortable.
If you plan on doing more than packing out trash or taking a couple of sandwiches I would not recommend any packing system which puts the entire weight of the load on a narrow harness-style saddle and especially one which features unpadded nylon bags. Anyone who has toted around a load of modern textbooks in a cheap backpack versus a nice one can tell you how much difference it makes! You want that load distributed over a wide area, well-padded and firmly anchored.
If you are uncomfortable investing in a proper pack saddle, I would get a mini-sized Western saddle and then put a set of big horse saddle bags over the seat and proceed that way. Anything that will comfortably distribute the load!
My injured mini much prefers being a driving horse but is still thrilled to get out for a walk in new places. He will happily lead me down the trail, always in a hurry to see what's around the next bend. That may be his future if he doesn't get back to driving!
I think as long as he has a job he will be happy and it sure beats picking up dog poop.
Leia