retainer vs workhorse

retainer

noun
  • A paid servant, especially one who has been employed for many years. 

  • A fee one pays to reserve the other's time for services. 

  • A device that holds teeth in position after orthodontic treatment. 

  • Any thing or person that retains. 

  • A dependent or follower of someone of rank. 

  • A retaining valve. 

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 retainer and workhorse occurred in a corpus of books? (source: Google Ngram Viewer )