fable vs untruth

fable and untruth both are nouns.

Word	Noun	Adjective	Verb	Adverb
fable	Yes	No	No	No
untruth	Yes	No	No	No

As nouns, untruth is a hypernym of fable; that is, untruth is a word with a broader meaning than fable:

Other hypernyms of fable include falsehood, falsity.

fable (noun)	untruth (noun)
a short moral story (often with animal characters)	a false statement
a story about mythical or supernatural beings or events
a deliberately false or improbable account