Mechagodzilla (aka Mecha-G, the Bionic Monster, Cosmic Monster, Kiryu): One day there is this atomic-powered reptile minding his own business in Japan in1974's Godzilla vs. Mechagodzilla. When some jerks (who look like warty-faced rejects from the Planet of the Apes) hailing from the Third Planet in the Black hole (it is nice for them to give us their address, just in case you want to drop them a line) decide to take over the Earth. What to do? Godzilla will easily roast them to little atomic piles, so they get this great idea. Make a robotic Godzilla. Which is actually pretty cool. So, cool in fact that other aliens will resurrect Mecha-G to fight the Big G in a sequel. Then later Mecha-G is reinvented as a good guy while Godzilla plays the heavy.
(Image courtesy of the Godzilla Wiki)