cirrus vs mare's tail

cirrus and mare's tail both are nouns.

Word	Noun	Adjective	Verb	Adverb
cirrus	Yes	No	No	No
mare's tail	Yes	No	No	No

As nouns, mare's tail is a hyponym of cirrus; that is, mare's tail is a word with a more specific, narrower meaning than cirrus:

cirrus: a wispy white cloud (usually of fine ice crystals) at a high altitude (4 to 8 miles)
mare's tail: a long narrow flowing cirrus cloud

cirrus (noun)	mare's tail (noun)
a slender flexible animal appendage as on barnacles or crinoids or many insects; often tactile	a long narrow flowing cirrus cloud
a wispy white cloud (usually of fine ice crystals) at a high altitude (4 to 8 miles)
usually coiled