Great Mazinger vs. Getter Robo G: The Great Space Encounter

When a UFO appears over Japan, both the heroes of Great Mazinger and Getter Robo want to investigate it first, out of a sense of rivalry. Getter finds it first since the Mazinger robot is being repaired. However they are unable to defeat the apparently living vehicle. It had already dropped a metal-eating monster on Japan named Gilgilgan and Great Mazinger confronts it only to be badly beaten. (wikipedia)