还是得根据语境翻译，beast of burden 是个短语，驮兽（如马、驴等）
维基百科的解释：
1.A draught animal that carries or pulls heavy loads, such as a donkey, mule, llama, camel or ox.
2.(by extension) Any domesticated animal trained to perform tasks for humans, such as a herding dog or trained falcon.
