monster vs boogeyman

monster and boogeyman both are nouns.

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

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

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

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

monster (noun)	boogeyman (noun)
(medicine) a grossly malformed and usually nonviable fetus	an imaginary monster used to frighten children
a cruel wicked and inhuman person
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