pes vs pes planus

pes and pes planus both are nouns.

Word	Noun	Adjective	Verb	Adverb
pes	Yes	No	No	No
pes planus	Yes	No	No	No

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

pes: the part of the leg of a human being below the ankle joint
pes planus: a foot afflicted with a fallen arch; abnormally flattened and spread out

Other hyponyms of pes include flatfoot, splayfoot.

pes (noun)	pes planus (noun)
the part of the leg of a human being below the ankle joint	a foot afflicted with a fallen arch; abnormally flattened and spread out