young mammal vs calf

young mammal and calf both are nouns.

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

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

young mammal (noun)	calf (noun)
any immature mammal	young of domestic cattle
	young of various large placental mammals e.g. whale or giraffe or elephant or buffalo
	the muscular back part of the shank
	fine leather from the skin of a calf