Don’t always look like this.
At first, they look like this:
They are pufferfish. And their body contains tetrodotoxin and saxitoxin.
These chemicals can easily cause death in humans, particularly if you consume them. The poison will attack your nervous system, causing dizziness and numbness, and eventually the inability to walk. The poison in a pufferfish is exponentially more powerful than cyanide.
When predators choose to eat them, they often spit them out really quickly, because the pufferfish intentionally tastes horrible, and the poison will create a strong reaction in the predator.
Also, they have extremely sharp and powerful teeth that can cut through your skin easily.. Despite being super cute.