mermaid vs imaginary creature

mermaid and imaginary creature both are nouns.

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

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

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

Other hypernyms of mermaid include imaginary being.

mermaid (noun)	imaginary creature (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