beast of burden
From Longman Dictionary of Contemporary EnglishRelated topics: Animalsbeast of burdenˌbeast of ˈburden noun [countable] old use HBAan animal that does heavy workExamples from the Corpusbeast 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.