1911 Encyclopædia Britannica/Sumpter

SUMPTER, a pack-horse or mule, a beast for carrying burdens, particularly for military purposes. There were two words once in use, which in sense, if not in form, have coalesced. These are “sommer” or “summer” and “sumpter.” The first comes through the Old French sommier, a pack-horse, the other through sommetier, a pack-horse driver. Both come ultimately from Late Lat. salma, from sagma, a pack, burden, Old French somme, saume; Greek σάγμα, burden, σάττειν, to load. “Sumpter” in the sense of a driver of a pack-horse is rare, and the word is always joined with another explanatory word.