Deputy Weld is a robot guard of Megaton in Fallout 3. He guards the main gate and has been programmed by Lucas Simms to say classic "wild-west" phrases. If Deputy Weld is destroyed, he'll be replaced by Megaton's other robot guard; Deputy Steel who currently guards the Megaton Armory.
If Megaton is destroyed during The Power of the Atom quest, Deputy Weld's head will survive the blast. It's laying on the floor near the entrance and it's still welcoming you to Megaton.