foal vs stud horse

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. 

stud horse

noun
  • A stallion; a male horse kept as a stud. 

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