thrall vs workhorse

thrall

noun
  • One who is enslaved or under mind control. 

  • The state of being under the control of another person. 

  • A shelf; a stand for barrels, etc. 

verb
  • To make a thrall; enslave. 

workhorse

noun
  • Anyone or anything that does a lot of work; something or someone who works consistently or regularly. 

  • A horse used primarily for manual labor; a draft horse. 

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