mermaid vs monkfish

mermaid

noun
  • A mythological creature with a woman's head and upper body, and a tail of a fish. 

  • Coloured a brilliant turquoise. 

monkfish

noun
  • Any large bottom-dwelling anglerfish of the genus Lophius, such as Lophius piscatorius, of the Atlantic, having a large head and mouth. 

  • Angel sharks of the genus Squatina. 

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