Part 2 : A little bit of family history
The yellow boxfish (Ostracion cubicus) lives in the Pacific and Indian ocean. It is part of the Genus Ostracion, which is composed of eight species of box- and trunkfish, all so-called because of their rectangular shape.
This genus, in turn, is part of the family ostraciidae, which accounts for all matter of square fish, called trunkfish, boxfish, cowfish or cofferfish, probably in accordance to which square object was most common in the cultures that first discovered these fishes.
In addition to the squareness of it's members (often more pronounced in the adults than in younger fish), the genus ostraciidae is known for it's hard, bony scales, as well as for the neurotoxins most of it's members are capable of secreting. This, in addition to the hard scales, could explain why the boxfish is still here today : not many predators would be able to devour such an ill-tasting, and probably rather crunchy meal.
So, it seems that the boxfish has taken some well thought-out precautions against extinction. Solid armor and poison always help you stay alive (if you don't believe me, just watch "Game of Thrones"). But even though we know why the boxfish managed to stay, there is one thing it does not explain : how did it come to exist in the first place ?