pes vs flatfoot

pes and flatfoot both are nouns.

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

As nouns, flatfoot is a hyponym of pes; that is, flatfoot 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
flatfoot: a foot afflicted with a fallen arch; abnormally flattened and spread out

Other hyponyms of pes include pes planus, splayfoot.

pes (noun)	flatfoot (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
	a policeman who patrols a given region