There's an octopus that can kill you in one bite - there is no anti-venom.

It's called the blue-ringed octopus, and it lives in tide pools and coral reefs in the Pacific and Indian Oceans as well as around Japan and Australia. It is recognized as one of the world’s most venomous marine animals. 

Though it is generally docile, and hunts only small crabs, hermit crabs, and shrimp, it may bite attackers if provoked. It carries enough venom to kill 26 people within minutes, and still no anti-venom exists. The venom contains tetrodotoxin, which is 10,000 times more toxic than cyanide.

Tetrodotoxin blocks sodium channels, causing motor paralysis and respiratory arrest. First aid treatment exists, however. Put simply, it is placing pressure on the wound and utilizing artificial respiration. A bag valve mask or medical ventilator can be sufficient to keep the victim alive until the venom is neutralized by the body. People who can live through the first 24 hours generally recover completely. 

(Source)