pen vs cow pen

pen and cow pen both are nouns.

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

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

pen (noun)	cow pen (noun)
female swan	a pen for cattle
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)	cow pen (verb)
produce a literary work