From Longman Dictionary of Contemporary Englishbeast of burdenˌbeast of ˈburden noun [countable] old use HBAan animal that does heavy work
Examples from the Corpus
beast of burden• A wolf killed and ate his beast of burden.• Will development ever again be the beast of burden pulling the region out of economic malady?• The donkey is the beast of burden, and windmills, originally used for grinding maize, abound. 