There are several synonyms for the term "pack mules" which can be used to describe animals or humans who carry heavy loads. The first synonym is "beasts of burden" which refers to animals like horses, donkeys, and mules that are used to carry loads. Another synonym is "porters" which can refer to humans who carry loads on their backs or shoulders. "Sherpas" are a specific type of porter who are renowned for their skill in carrying loads up mountainous terrain. "Transporters" and "carriers" are also synonyms for pack mules, and they can refer to both animals and humans who carry heavy loads. Overall, these synonyms illustrate the versatility of individuals and animals who are used to transport goods and supplies across varying terrains.