When talking about nature’s most badass skeletons (a conversation in which I may be talking to myself), surely the Porcupinefish (Diodon nicthemerus) deserves a mention.
The Porcupinefish are best known for their effective defense mechanism in which they inflate their bodies by swallowing water or air. Some species are even poisonous!
According to the Florida Museum, the spines that cover their body are not technically bones but ‘modified scales’. Further, the spines only sick out when the fish is inflated and threatened. At all other times the spines lay flat against the body.