Title. In addition to being able to inflict poison when you get close to one when it's puffed up, it deals a tiny bit of damage separate from the poison. If you're already at a half heart (from being poisoned down to a half heart, for example), and you go near a puffed up pufferfish, you die, but the death message doesn't have a proper name for the pufferfish, only "entity.minecraft.pufferfish". Similarly, if you check the Mobs section of the Statistics screen, it has that same name instead of a proper name.