mustang vs packhorse

mustang

noun
  • A small, hardy, naturalized (feral) horse of the North American west. 

  • A merchant marine who joined the U.S. Navy as a commissioned officer during the American Civil War. 

  • A commissioned officer who started military service as an enlisted person. 

verb
  • To hunt mustangs (wild horses), either for military use or for slaughter as pet food. 

packhorse

noun
  • A horse used as a pack animal. 

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