sheep pen vs pen

sheep pen and pen both are nouns.

Word	Noun	Adjective	Verb	Adverb
sheep pen	Yes	No	No	No
pen	Yes	No	Yes	No

As nouns, pen is a hypernym of sheep pen; that is, pen is a word with a broader meaning than sheep pen:

sheep pen (noun)	pen (noun)
a pen for sheep	female swan
	a writing implement with a point from which ink flows
	an enclosure for confining livestock
	a correctional institution for those convicted of major crimes
	a portable enclosure in which babies may be left to play

sheep pen (verb)	pen (verb)
	produce a literary work