common dolphin vs hellbender

common dolphin

noun
  • One of two species of dolphin in the genus Delphinus. 

hellbender

noun
  • A giant salamander (genus Cryptobranchus) of North America that inhabits large, swiftly flowing streams with rocky bottoms. 

  • A reckless, headstrong person. 

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