mustang vs wild horse

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. 

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 mustang and wild horse occurred in a corpus of books? (source: Google Ngram Viewer )