monster vs bugaboo

monster and bugaboo both are nouns.

Word	Noun	Adjective	Verb	Adverb
monster	Yes	No	No	No
bugaboo	Yes	No	No	No

As nouns, bugaboo is a hyponym of monster; that is, bugaboo is a word with a more specific, narrower meaning than monster:

monster: an imaginary creature usually having various human and animal parts
bugaboo: an imaginary monster used to frighten children

Other hyponyms of monster include bogeyman, booger, boogeyman, bugbear, mythical creature, mythical monster, yokai.

monster (noun)	bugaboo (noun)
(medicine) a grossly malformed and usually nonviable fetus	a source of concern
a cruel wicked and inhuman person	an imaginary monster used to frighten children
a person or animal that is markedly unusual or deformed
an imaginary creature usually having various human and animal parts
someone that is abnormally large and powerful