draft horse vs pack horse

draft horse

noun
  • A horse used for muscular work, other than carrying a rider and/or load, such as pulling a cart, plow or log. 

  • A horse of a race specifically bred for such work, heavier and stronger than a mount. 

pack horse

noun
  • A horse used for carrying luggage. 

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