This ought to be up PZ Myers's alley: the blue-ringed octopus.
Cute, isn't it? This little creature's venom can kill an adult human in minutes; its distinctive blue rings are not visible until the octopus is about to attack; and it illustrates Yoda's famed maxim that size matters not. The blue-ringed octopus is about the size of a golf ball.
More linkage from the MeFi thread wherein I learned about this little beastie.