mayfish vs killifish

mayfish and killifish both are nouns.

Word	Noun	Adjective	Verb	Adverb
mayfish	Yes	No	No	No
killifish	Yes	No	No	No

As nouns, killifish is a hypernym of mayfish; that is, killifish is a word with a broader meaning than mayfish:

mayfish: black-barred fish of bays and coastal marshes of the Atlantic and Gulf Coast of the United States
killifish: small mostly marine warm-water carp-like schooling fishes; used as bait or aquarium fishes or in mosquito control

mayfish (noun)	killifish (noun)
black-barred fish of bays and coastal marshes of the Atlantic and Gulf Coast of the United States	small mostly marine warm-water carp-like schooling fishes; used as bait or aquarium fishes or in mosquito control