paddock vs pen

paddock and pen both are nouns.

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

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

paddock (noun)	pen (noun)
pen where racehorses are saddled and paraded before a race	female swan
	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

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