bannerfish

Short definition from Wiktionary

Noun

1. 1
   A large sailfish of the species Istiophorus platypterus, of the swordfish family, with a broad banner-like dorsal fin
2. 2
   Any of several species of butterflyfish in the genus Heniochus (order Chaetodontiformes) that superficially resemble the Moorish idol (Zanclus cornutus, order Acanthuriformes), though not closely related.