megathere vs megatheriid

megathere and megatheriid both are nouns.

Word	Noun	Adjective	Verb	Adverb
megathere	Yes	No	No	No
megatheriid	Yes	No	No	No

As nouns, megatheriid is a hypernym of megathere; that is, megatheriid is a word with a broader meaning than megathere:

megathere: gigantic extinct terrestrial sloth-like mammal of the Pliocene and Pleistocene in America
megatheriid: a large extinct ground sloth

Other hypernyms of megathere include megatherian, megatherian mammal.

megathere (noun)	megatheriid (noun)
gigantic extinct terrestrial sloth-like mammal of the Pliocene and Pleistocene in America	a large extinct ground sloth