Large Unicorn Tang, Naso unicornis, spikes extended (perhaps because I was a bit close), at Queen's Surf, Hawaii.
Here's a headshot showing the horn up close and personal. This fish is also known as the bluespine unicornfish.
Note that Unicornfish are in the family acanthuridae. They are also known as tangs or surgeonfish, for the sharp, knife-like spines at their tail that can slash you pretty badly if you attempt to grab them by the tail. In some surgeonfish species, the spines are venemous, although I have not, thankfully, experienced this firsthand. Most are peaceful herbivores, grazing in large schools along shallow reefs (although a few species also live on small invertebrates). I jokingly refer to the surgeonfish as the cows of the sea. In aquariums, they will strip a tank bare of macroalgae in a day or two...so, if you have a macroalgae problem, these are your fish. Noteably, the macroalgae also provides the tangs with needed micronutrients, preventing the pitting that you sometimes see in tank fish that are not fed an occassional meal of macroalgae.
