sheath vs natural covering

sheath and natural covering both are nouns.

Word	Noun	Adjective	Verb	Adverb
sheath	Yes	No	No	No
natural covering	Yes	No	No	No

As nouns, natural covering is a hypernym of sheath; that is, natural covering is a word with a broader meaning than sheath:

sheath: an enveloping structure or covering enclosing an animal or plant organ or part
natural covering: a natural object that covers or envelops

Other hypernyms of sheath include cover, covering.

sheath (noun)	natural covering (noun)
a protective covering (as for a knife or sword)	a natural object that covers or envelops
a dress suitable for formal occasions
an enveloping structure or covering enclosing an animal or plant organ or part