I did a quick search, mules are mentioned 75 different times. Like this one at random from Sept 1942: https://forestrydiary.com/page/019bd90a-f176-713f-9999-b14b6...

"Fix up my packs. Load the 2 mules with 225# each. Take the 2 loads to trail camp at Lake Everett, Unload. Have lunch with the Trail cook. Haze mules & ride to 7 1/2 PM."

Horses are mentioned 2586 times. That'd be a whole study on how they're used in the back country. (Edit: horse number is inflated since part of the diary form at one point asks for "Horse Mileage". Will have to refine search).