monster vs bogeyman

monster and bogeyman both are nouns.

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

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

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

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

monster (noun)	bogeyman (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