Pufferfish

Hit it, or pull it out of the water, and it inflates into a ball and bobs up to the surface. That is the entire behavior, and it is worth watching once before you decide what to do with it.

It is rare and tied to the Ocean, so nobody stumbles across one on the way somewhere else. If you want it for an aquarium or a display build, go out specifically to look, give the shoreline time to repopulate between passes, and bring something to carry it home in. The tooltip is a nod to a certain shark support group in Finding Nemo.