mythical being vs hero

mythical being and hero both are nouns.

Word	Noun	Adjective	Verb	Adverb
mythical being	Yes	No	No	No
hero	Yes	No	No	No

As nouns, hero is a hyponym of mythical being; that is, hero is a word with a more specific, narrower meaning than mythical being:

mythical being: an imaginary being of myth or fable
hero: (classical mythology) a being of great strength and courage celebrated for bold exploits; often the offspring of a mortal and a god

Other hyponyms of mythical being include halcyon, mythical creature, mythical monster, phoenix, houri, Amazon, Gog and Magog, golem.

mythical being (noun)	hero (noun)
an imaginary being of myth or fable	the principal character in a play or movie or novel or poem
	a large sandwich made of a long crusty roll split lengthwise and filled with meats and cheese (and tomato and onion and lettuce and condiments); different names are used in different sections of the United States
	a person distinguished by exceptional courage and nobility and strength
	someone who fights for a cause
	(classical mythology) a being of great strength and courage celebrated for bold exploits; often the offspring of a mortal and a god