beast of burden vs pack animal

beast of burden

noun
  • A draught animal, such as a donkey, mule, llama, camel, horse, or ox, that carries or pulls a load for the benefit of a human. 

  • Any domesticated animal trained to perform tasks for humans, such as a herding dog or trained falcon. 

pack animal

noun
  • A domesticated animal used to carry heavy items. 

  • A wild animal that lives and hunts in packs 

How often have the words beast of burden and pack animal occurred in a corpus of books? (source: Google Ngram Viewer )