courser vs dark horse

courser

noun
  • A racehorse or a charger. 

  • A person who practises coursing. 

  • Any of several species of bird in the genera Cursorius or Rhinoptilus of the family Glareolidae. 

  • A dog used for coursing. 

  • A stone used in building a course. 

  • A hunter. 

dark horse

noun
  • A horse whose capabilities are not known. 

  • A candidate for an election who is nominated unexpectedly, without previously having been discussed or considered as a likely choice. 

  • Someone who possesses talents or favorable characteristics that are not known or expected by others. 

  • Used other than figuratively or idiomatically: see dark, horse. 

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