monster vs yokai

monster and yokai both are nouns.

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

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

monster: an imaginary creature usually having various human and animal parts
yokai: Any of various supernatural monsters, sometimes shapeshifters, in Japanese folklore

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

monster (noun)	yokai (noun)
(medicine) a grossly malformed and usually nonviable fetus	Any of various supernatural monsters, sometimes shapeshifters, in Japanese folklore
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