cowlet vs freemartin

cowlet

noun
  • A young or little cow; a female calf. 

freemartin

noun
  • A female calf, born as twin with a bull calf, but sexually imperfect (often infertile). 

  • Any female animal born sterile or otherwise infertile. 

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