The aquarium had several brightly colored butterflyfish swimming gracefully.

butterflyfish 是复合名词，由 butterfly（蝴蝶）和 fish（鱼）组成。butterfly 形容这种鱼像蝴蝶一样色彩鲜艳，fish 表明这是一种鱼类。合起来指"蝴蝶鱼"，这是一种热带海洋鱼类，因体色艳丽、形似蝴蝶而得名。