beast of burden vs mammal

beast of burden

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

  • 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. 

mammal

noun
  • An animal of the class Mammalia, characterized by being warm-blooded, having hair and producing milk with which to feed its young. 

  • A vertebrate with three bones in the inner ear and one in the jaw. 

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