sheepdog vs shepherd dog

sheepdog

noun

  • A breed of dog used for guarding sheep. 

  • A breed of dog, used for herding sheep. 

  • A police officer. 

verb

  • To chaperon or shepherd. 

shepherd dog

noun

  • A sheepdog. 

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