pen vs sheep pen

pen and sheep pen both are nouns.

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

As nouns, sheep pen is a hyponym of pen; that is, sheep pen is a word with a more specific, narrower meaning than pen:

pen (noun)	sheep pen (noun)
female swan	a pen for sheep
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

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