I think you found a pufferfish or blowfish... and I think you're exactly right that you're glad you didn't step on it. Here's some information on what I think your beautiful/creepy beach monster might have been. Great post!
Pufferfish can inflate into a ball shape to evade predators. Also known as blowfish, these clumsy swimmers fill their elastic stomachs with huge amounts of water (and sometimes air) and blow themselves up to several times their normal size.
Some species also have spines on their skin to ward off predators. Even if a predator gobbles up a puffer before it inflates, it won't enjoy the snack. Most pufferfish contain a toxic substance that makes them foul tasting and potentially deadly to other fish. The toxin is deadly to humans. There is enough poison in one pufferfish to kill 30 adult humans, and there is no known antidote.
RE: Monster On The Beach