pen vs writing implement

pen and writing implement both are nouns.

Word	Noun	Adjective	Verb	Adverb
pen	Yes	No	Yes	No
writing implement	Yes	No	No	No

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

pen (noun)	writing implement (noun)
female swan	an implement that is used to write
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)	writing implement (verb)
produce a literary work