pack horse vs pommel horse

pack horse

noun
  • A horse used for carrying luggage. 

pommel horse

noun
  • An athletic event in which the pommel horse is used. 

  • An apparatus for gymnastic exercises with a cylindrical body covered with leather and two upright pommels, as hand grips, near the centre; held upright with adjustable legs. 

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