young mammal vs kit

young mammal and kit both are nouns.

Word	Noun	Adjective	Verb	Adverb
young mammal	Yes	No	No	No
kit	Yes	No	Yes	No

As nouns, kit is a hyponym of young mammal; that is, kit is a word with a more specific, narrower meaning than young mammal:

young mammal (noun)	kit (noun)
any immature mammal	young of any of various fur-bearing animals
	gear consisting of a set of articles or tools for a specified purpose
	a case for containing a set of articles

young mammal (verb)	kit (verb)
	supply with a set of articles or tools