Instead of saddle panniers which are trouble to load, unload, and balance, look at the MountainRidgeGear.com XL Hunter panniers that can be used on a saw buck or riding saddle. Most riding saddles except for mules do not have britching and that can get you in trouble going downhills with saddle panniers.
Here is a great web site for all kinds of packing .
https://packsaddleshop.com/pages/horse-packing.Good Facebook page. Packing In on Horses and Mules- BackCountry Stock Users