bug-a-boo vs specter

bug-a-boo

noun
  • Any imagined fear or threat, or a fear presumed larger than it really is. 

  • A mythical, nocturnal creature; a hobgoblin. 

specter

noun
  • A threatening mental image. 

  • Any of certain species of dragonfly of the genus Boyeria, family Aeshnidae. 

  • A ghostly apparition, a phantom. 

How often have the words bug-a-boo and specter occurred in a corpus of books? (source: Google Ngram Viewer )