beast of burden

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An animal, such as a donkey, ox, or elephant, used for transporting loads or doing other heavy work.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A draught animal that carries or pulls heavy loads, such as a donkey, mule, or ox.
• noun by extension Any domesticated animal trained to perform tasks for humans, such as a herding dog or trained falcon.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an animal such as a donkey or ox or elephant used for transporting loads or doing other heavy work