The blue-ringed octopus is the deadliest of all cephalopods (a group of mollusks which also includes squids and cuttlefish).
When it’s agitated, iridescent blue rings appear and pulsate, and its brown patches darken:
If you touch it, it’ll bite you and inject you with venom. In fact, this octopus carries enough venom to kill 26 adult humans within minutes!
On another note, have you ever heard of a blue dot sea hare?