The story
From butterfly + fish, from their butterfly-like appearance and bright colours.
Etymology from Wiktionary, CC BY-SA 4.0.
butterflyfish
Today it means: Any of a group of conspicuous tropical marine fish of the family Chaetodontidae; the bannerfish and coralfish are also included in this group. Butterflyfish are fairly small, mostly from 12 to 22 cm (c.7-9 in) in length. Full definition →