croupe vs crupper

croupe

crupper

verb
  • To fit with a crupper; to place a crupper upon. 

noun
  • A strap, looped under a horse's tail, used to stop a saddle from slipping. 

  • The buttocks or rump, especially of a horse. 

  • A piece of armour covering the hindquarters/buttocks of a horse. 

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