sheepskin vs leather

sheepskin and leather both are nouns.

Word	Noun	Adjective	Verb	Adverb
sheepskin	Yes	No	No	No
leather	Yes	No	Yes	No

As nouns, leather is a hypernym of sheepskin; that is, leather is a word with a broader meaning than sheepskin:

sheepskin: tanned skin of a sheep with the fleece left on; used for clothing
leather: an animal skin made smooth and flexible by removing the hair and then tanning

sheepskin (noun)	leather (noun)
a document certifying the successful completion of a course of study	an animal skin made smooth and flexible by removing the hair and then tanning
tanned skin of a sheep with the fleece left on; used for clothing
skin of a sheep or goat prepared for writing on

sheepskin (verb)	leather (verb)
	whip with a leather strap