une bête de somme
Une bête de somme means a workhorse or a beast of burden. Back in the 12th century, “une somme” meant the load that a horse or a mule could carry. Then, in the 16th century, the term “bête de somme” arrived. These days, the more common word for burden is “un fardeau,” but the term “bête de somme” survives in its original form.