Old Dan (the Devil) was often the sturborn pack mule. I believe the mules outnumbered the horses in the Old West.   When broke to the saddle they could out run a horse ( though not out-turn ) and their endurance was better than a horse and more sure-footed. As someone posted above, if somebody was traveling to new digs your grub and gear had to travel on several animals and if you well off enough to own a wagon (which was a rough go where no roads existed) you still had to have a team and alternate animals.