headgear vs topknot

headgear and topknot both are nouns.

Word	Noun	Adjective	Verb	Adverb
headgear	Yes	No	No	No
topknot	Yes	No	No	No

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

headgear: clothing for the head
topknot: headdress consisting of a decorative ribbon or bow worn in the hair

headgear (noun)	topknot (noun)
clothing for the head	showy crest or knot of hair or feathers
stable gear consisting of any part of a harness that fits about the horse's head	headdress consisting of a decorative ribbon or bow worn in the hair
the hoist at the pithead of a mine