mermaid vs imaginary being

mermaid and imaginary being both are nouns.

Word	Noun	Adjective	Verb	Adverb
mermaid	Yes	No	No	No
imaginary being	Yes	No	No	No

As nouns, imaginary being is a hypernym of mermaid; that is, imaginary being is a word with a broader meaning than mermaid:

mermaid: half woman and half fish; lives in the sea
imaginary being: a creature of the imagination; a person that exists only in legends or myths or fiction

Other hypernyms of mermaid include imaginary creature.

mermaid (noun)	imaginary being (noun)
half woman and half fish; lives in the sea	a creature of the imagination; a person that exists only in legends or myths or fiction