Blue-Ringed Octopus (Hapalochlaena)
These sneaky octopi are sand-coloured and hide in shells, so you are unlikely to see one of these until it has already bitten you. When agitated, the bright blue rings as pictured appear on the skin. Unfortunately, by this stage it is likely too late for you, as blue ringed octopus bites are always fatal. To this day there is no known antidote.