A reef fish (Rhinecanthus rectangulus) sometimes known as the "unofficial" state fish of Hawaii. Also known as the pig-nosed triggerfish, reef triggerfish, rectangular triggerfish, or humuhumu triggerfish.
The name humu-humu-nuku-nuku-apua'a supposedly means "fish that sounds like a pig" in Hawaiian, and when brought out of the water, it does indeed make grunting noises. Like other triggerfish, the forward spine of its dorsal fin, slightly behind the eye, is strong, sharp, and can be locked in a rigid erect position for defense. (This is where the 'trigger' in the name comes from). A related species, the picasso triggerfish (Rhinecanthus aculeatus), is sometimes referred to by the same (long) name.
Trivia guaranteed to impress only an ichthyologist: the humu-humu-nuku-nuku-apua'a does not have the longest name of all state fishes. That honor (?) goes to Utah's state fish, the Bonneville cutthroat trout. (This only works if you believe that the word "triggerfish" is redundant and can be omitted.)