A pack animal or beast of burden is an individual or type of working animal used by humans as means of transporting materials by attaching them so their weight bears on the animal's back, in contrast to draft animals which pull loads but do not carry them.
The term pack animal is traditionally used in contrast to draft animal, which is a working animal that typically pulls a load behind itself (such as a plow, a cart, a sled or a heavy log) rather than carrying cargo directly on its back. For instance, sled dogs pull loads but do not normally carry them, while working elephants have been used for centuries to haul logs out of forests.
While traditional usage of pack animals by nomadic tribespeople is declining, a new market is growing in the tourist expeditions industry in regions such as the High Atlas mountains of Morocco, allowing visitors the comfort of backpacking with animals. The use of pack animals "is considered a valid means of viewing and experiencing" some National Parks in America, subject to guidelines and closed areas.
In the 21st century, special forces have received guidance on the use of horses, mules, llamas, camels, dogs, and elephants as pack animals.
Load carrying capacity
The load for oxen is their body weight (1500-2500 pounds) at walking pace and 2X for short hauls. 
The maximum load for a camel is roughly 300 kg.
Yaks are loaded differently according to region. In Sichuan, 75 kg is carried for 30 km in 6 hours. In Qinghai, at 4100 m altitude, packs of up to 300 kg are routinely carried, while up to 390 kg is carried by the heaviest steers for short periods.
Llamas can carry roughly 1/4 of their body weight, so an adult male of 200 kg can carry some 50 kg.
Loads for equids are disputed. The US Army specifies a maximum of 20 percent of body weight for mules walking up to 20 miles a day in mountains, giving a load of up to about 150 kg. However an 1867 text mentioned a load of up to 800 pounds (about 360 kg). In India, the prevention of cruelty rules (1965) limit mules to 200 kg and ponies to 70 kg.
Reindeer can carry up to 40 kg for a prolonged period in mountains.
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