fern ally vs horsetail

fern ally and horsetail both are nouns.

Word	Noun	Adjective	Verb	Adverb
fern ally	Yes	No	No	No
horsetail	Yes	No	No	No

As nouns, horsetail is a hyponym of fern ally; that is, horsetail is a word with a more specific, narrower meaning than fern ally:

fern ally: pteridophytes of other classes than Filicopsida
horsetail: perennial rushlike flowerless herbs with jointed hollow stems and narrow toothlike leaves that spread by creeping rhizomes; tend to become weedy; common in Northern Hemisphere; some in Africa and South America

Other hyponyms of fern ally include whisk fern, club-moss, club moss, lycopod, quillwort.

fern ally (noun)	horsetail (noun)
pteridophytes of other classes than Filicopsida	perennial rushlike flowerless herbs with jointed hollow stems and narrow toothlike leaves that spread by creeping rhizomes; tend to become weedy; common in Northern Hemisphere; some in Africa and South America