A group of mules is called a string, a term with deep roots in the working history of these hybrid animals. Mules, the offspring of a male donkey and a female horse, were prized for centuries as pack animals and draft animals precisely because they combine a horse’s strength with a donkey’s endurance and surefootedness.

“String” reflects how mules were often worked, tied nose-to-tail in a line so a single handler could lead a whole team through narrow mountain trails, mining routes, or military supply lines where wagons couldn’t go. The term is especially common in ranching and Western U.S. vocabulary, where a “string of mules” or “string of pack animals” still shows up in trail-riding and outfitting contexts today.

Mules also go by other collective nouns depending on the source, including a pack of mules and a span of mules (specifically for a pair harnessed together), but “string” remains the most evocative, calling to mind the long single-file lines that carried goods across difficult terrain for generations.