A pack animal is a beast of burden 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 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 draft 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".
In addition to referring to a backpack ("burden"), a pack also means a large number of wild animals of the same kind. Thus "pack animal" is occasionally used to mean animals who live in packs (i.e., social animals). In this sense, dogs are also classified as pack animals because they live, feed, and travel as a group.