Pack horses and pack mules each have specific characteristics that make them better suited to certain packing contexts. Mules — the offspring of a horse mare and a donkey jack — are generally considered superior pack animals for most serious backcountry applications, and the preponderance of professional outfitting operations in the American West rely on mules as their primary pack animals.
Mules have harder, more compact hooves than horses that hold up better on rocky terrain. They have longer ears that help regulate body temperature in hot conditions, and their physiology tends toward greater efficiency — a mule will typically eat less than a horse of comparable size while doing similar work. Mules are also widely considered more sure-footed and better at self-preservation — they are less likely than horses to panic and run into danger.
Pack horses are more widely available, often at lower cost, and can work extremely well in packing applications when properly selected and trained. Quarter Horses, draft crossbreeds, and cold-blooded types with broad, strong backs and calm temperaments are popular pack horse breeds. Horses tend to be more sensitive and reactive than mules, which means their training must emphasize desensitization to packing equipment more thoroughly than mule training typically requires.