beast of burden vs wild horse

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. 

wild horse

noun
  • A feral horse. 

  • An untamed horse. 

  • A member of the species Equus ferus, sometimes excluding the domesticated horse. 

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