Blue Ring Octopus
The blue ring octopus may be small and cute looking, but it is one of the most venomous species whose toxins can easily kill a grown man in no time. This octopus uses the same toxin, tetrodotoxin, for its self-defense as the puffer fish.
It packs another fatal one only for hunting. This other toxin is harmless to humans, but tetrodotoxin is so strong a single dose in can knock down 26 humans at the same time.