The Collaborative International Dictionary of English v.0.48:

Bugaboo \Bug`a*boo"\, Bugbear \Bug"bear`\, n. [See Bug.]
   Something frightful, as a specter; anything imaginary that
   causes needless fright; something used to excite needless
   fear; also, something really dangerous, or an imaginary
   monster, used to frighten children, etc. "Bugaboos to fright
   ye." --Lloyd.
   [1913 Webster]

         But, to the world no bugbear is so great
         As want of figure and a small estate.    --Pope.
   [1913 Webster]

         The bugaboo of the liberals is the church pray. --S. B.
                                                  Griffin.
   [1913 Webster]

         The great bugaboo of the birds is the owl. --J.
                                                  Burroughs.
   [1913 Webster]

   2. a source of concern; as, the old bugaboo of inflation
      still bothers them.
      [WordNet 1.5]

   Syn: Hobgoblin; goblin; specter; ogre; scarecrow; bogeyman;
        boogeyman; booger.
        [1913 Webster]

WordNet (r) 3.0 (2006):

bugaboo
    n 1: an imaginary monster used to frighten children [syn:
         bogeyman, bugbear, bugaboo, boogeyman, booger]
    2: a source of concern; "the old bugaboo of inflation still
       bothers them"