Most poisonous fish on Earth
It can easily kill you if you step on it, injecting its venom deep inside your foot. If not treated promptly, the poison will kill you. This video by Smarter Every Day shows how this simple killing machine works.
If you go to Australia, watch out for these bastards. They would be underwater, hiding under the sand or camouflaged over rocks, but also outside water, where they can survive for 24 hours. Apparently, stonefish antivenom is the second-most administered in Australia.