10. Blue-ringed octopus
The last animal on the list may catch you off-guard, which is precisely why it is so dangerous. Unbeknownst to many, the blue-ringed octopus is highly venomous, with an LD50 rating of 0.300 and venom containing powerful neurotoxins called maculotoxins.
Responsible for two known deaths in Australia, their fast-acting venom can induce symptoms (muscular weakness, difficulty breathing and paralysis) within 10 minutes and death within 30 minutes, due to a complete lack of oxygen to the brain, unless CPR is commenced quickly. For such a small creature (25g), the blue-ringed octopus is powerful enough to kill 26 humans with one bite; but bites are uncommon, with no known deaths since the 1960s. There is no antidote.