Some people think dragonflies are poisonous. They are not (though their larvae might be dangerous if you happen to be a tadpole).
According to a recent BBC Radio 4 programme about adders, people used to believe they could change to and from being adders/vipers - an ancient form of shape-shifting? So this could account for the mistaken belief that they are poisonous.

Apparently, people also used to believe that barnacle geese hatched from barnacles, and therefore counted as fish so you could eat them on a Friday.