noun (n)

• an imaginary monster used to frighten children(noun.person)

  Synonym:
  bogeyman, booger, boogeyman, bugbear
  

  source: wordnet30
• a source of concern(noun.cognition)

  Example:
  The old bugaboo of inflation still bothers them.
  

  source: wordnet30
• Something frightful, as a specter; anything imaginary that causes needless fright; something used to excite needless fear; also, something really dangerous, used to frighten children, etc.(noun)

  source: webster1913