Pack mule is a term that refers to an animal that is trained to carry heavy loads on its back. However, there are several synonyms that can be used to describe this type of animal. For instance, another term that is often used is pack horse, which implies an equine animal that is typically used for carrying packs. Another synonym is burden bearer, which could refer to any animal or human being that carries a heavy load. Additionally, the term pack animal can be used to refer to any creature that is trained to carry loads, including donkeys, llamas, camels, and oxen. Overall, there are several synonyms for pack mule that can be used to describe animals that are trained for carrying heavy loads.