A pack animal or beast of burden is a working animal used by humans as means of transporting materials by attaching them so their weight bears on the animal's back; the term may be applied to either an individual animal or a species so employed. The term pack animal is sometimes used in contrast to draft animal, which is a working animal that typically pulls a load behind itself (such as a plow or a wheeled cart) rather than carrying cargo directly on its back.
Many ungulate species are traditional pack animals, including elephants, camels, the yak, reindeer, goats, water buffalo and llama, and many of the domesticated Equidae (horse family).
The term is not routinely applied to humans carrying loads on their backs except to make a pejorative point about the injustice of so employing them, or about the privation that usually occasions accepting such work without explicit coercion. (The 1978 Rolling Stones song "Beast of Burden" refers to a sense of abuse, accepted within a romantic relationship.) Nevertheless, from a physical point of view, certainly many considerations apply equally to human and other pack animals, without considering the range of social conditions ranging through slaves, abused women and children, Himalayan and African natives employed as expedition porters, vacationing students whose duties as staff of mountaineering huts include packing heavy loads of supplies up steep slopes, and purely recreational hikers and backpackers including both short-trip ones, and long-trip backpackers who court injury and emaciation in carrying their heavy loads.
Another unconventional form of pack animal may be the dogs that are brought along on hikes carrying their own supply of drinking water and snacks on their backs, whether to provide them more exercise, or in pursuit of a hiker's ethic of "everyone carries his own gear".