There are many, many different species of butterfly, so in order to provide some kind of answer let's consider only the Monarch butterfly. Monarchs feed on milkweed, which contains a compound that is...
Snakes do not have poison, they have venom. Some venomous snakes are taipans, Gaboon vipers, Mangshan vipers, Eyelash crested vipers, and Sawscaled vipers. Those are the coolest in my opinion.