pen vs pigpen

pen and pigpen both are nouns.

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

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

pen (noun)	pigpen (noun)
female swan	a pen for swine
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)	pigpen (verb)
produce a literary work