beast of burden

American Heritage® Dictionary of the English Language, Fourth Edition

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

Wiktionary

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

GNU Webster's 1913

1. n. an animal employed in carrying burdens.

WordNet 3.0

1. n. an animal such as a donkey or ox or elephant used for transporting loads or doing other heavy work