thresher vs thresher shark

thresher

noun
  • Any of several large pelagic sharks of the genus Alopias, which have a very long tail; more commonly called thresher sharks. 

  • Anything or anyone that threshes. 

  • A now-obsolete hand tool for threshing, also called a flail. 

  • A modern farm machine for threshing grain, now a part of combine harvesters rather than a separate implement. 

thresher shark

noun
  • Any of the species of sharks in the genus Alopias, three of which are extant. 

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