#3: The Cylons

How deadly are they? They destroyed the twelve colonies of humanity, committing genocide on a scale unimaginable to everyone except Glen A. Larson and Ron Moore.
Who's responsible? It depends on which version you're asking about. The original series had the robots built by a dying alien race also called cylons, whereas the recent reboot gave them a backstory not unlike The Matrix's robot overlords (Built by humans as worker drones before rebelling and starting war).
Last seen... restarting the human race by populating Earth, millions of years ago, in this year's Battlestar Galactica season finale.

Next: Planet-Eating Robots!