mustelid vs polecat

mustelid and polecat both are nouns.

Word	Noun	Adjective	Verb	Adverb
mustelid	Yes	No	No	No
polecat	Yes	No	No	No

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

mustelid: fissiped fur-bearing carnivorous mammals
polecat: dark brown mustelid of woodlands of Eurasia that gives off an unpleasant odor when threatened

mustelid (noun)	polecat (noun)
fissiped fur-bearing carnivorous mammals	dark brown mustelid of woodlands of Eurasia that gives off an unpleasant odor when threatened
	American musteline mammal typically ejecting an intensely malodorous fluid when startled; in some classifications put in a separate subfamily Mephitinae