foal vs yearling

foal

noun
  • A young horse or other equine, especially just after birth or less than a year old. 

  • A young boy who assisted the headsman by pushing or pulling the tub. 

verb
  • To give birth to (a foal); to bear offspring. 

yearling

noun
  • A racehorse that is considered to be one year old until a subsequent January 1st. 

  • A sophomore at West Point military academy. 

  • An animal that is between one and two years old; one that is in its second year (but not yet two full years old). 

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