The term "pack animal" refers to any type of animal that is trained or able to carry a load on its back, such as a mule, donkey, horse, or even an elephant. These animals are often used for transportation and carrying goods over long distances in areas where other forms of transportation may not be practical or available. The phrase "pack animal" is also sometimes used more broadly to refer to any creature that carries its young on its back, such as certain species of birds or reptiles.