Morgan's mule vs equine

Morgan's mule

noun
  • The horse (instrument of punishment). 

equine

noun
  • Any horse or horse-like animal, especially one of the genus Equus. 

adj
  • Of or relating to any member or members of the genus Equus. 

  • Of or relating to a horse or horses. 

How often have the words Morgan's mule and equine occurred in a corpus of books? (source: Google Ngram Viewer )