Out of primordial depths to destroy the world!
In It Came From Beneath The Sea (1955), a giant radioactive octopus teaches the city of San Francisco a lesson about the importance of nuclear safety. By way of a violent rampage, of course. The good news (for viewers, at least) is that the octopus came courtesy of Ray Harryhausen.
Of course, legend has it that the producer only gave Harryhausen enough money to animate six of the octopus' tentacles. Click through for the full poster art, and for a clip that will let you count Harryhausen's handiwork for yourself.