The blue-ringed octopus, above, has been cited as a producer of one of the top 33 most deadly substances on Earth. By, uh, TheToyZone.com:

Its brown-speckled coloration is distinguished by the blue bands around its tentacles which give it its name. This small animal can be very dangerous, but it gives a warning signal in advance. When the bands around the tentacles glow blue, poison is about to be released. The blue-ringed octopus almost never bothers humans unless provoked, but since it is so small, many people think it is harmless. No more than six inches across, this venomous sea creature has a true octopus shape.

Case Study: In 1967, a young soldier held an octopus on the back of his hand for a minute or two and had no sensation of a bite. But after putting the creature down, he noticed blood on his hand. Just a few minutes later he felt a prickling sensation around his mouth, which rapidly spread over his whole body. Within just fifteen minutes, he was almost completely paralyzed and barely able to breathe. After an hour, this healthy young sailor began to vomit and soon went into convulsions. One hour after the bite, he was taken to the hospital, still breathing, fully conscious but completely paralyzed with no muscle tone and no reflexes. He couldn’t even talk. In another hour, stopped breathing and was put on a heart-lung machine. Miraculously, he eventually survived the ordeal.

They give no citations or anything ,so I don’t know where the information comes from, or what the toxicity rankings are based on. I presume something like LD50?

Interesting read, though — especially noting just how much of the list is dominated by plants, rather than animals.