pen vs fountain pen

pen and fountain pen both are nouns.

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

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

pen: a writing implement with a point from which ink flows
fountain pen: a pen that is supplied with ink from a reservoir in its barrel

pen (noun)	fountain pen (noun)
female swan	a pen that is supplied with ink from a reservoir in its barrel
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)	fountain pen (verb)
produce a literary work