Redmoon came from the moon, and some time later Erabus appeared. The Japanese Self Defense Force thought they could lure the two kaiju together so they could defeat each other, but they turn out to be two members of the same species and of opposite genders. Later, they have a son dubbed Hafun, who is killed later on. Stricken with grief, Redmoon and Erabus go berserk and attack Japan, but Godzilla intervenes and defeats them.
For a relatively long amount of time, it was thought that Redmoon and his family were to simply be portrayed by reusing the suits from the Tsuburaya film Daigoro vs. Goliath. However, Daigoro vs. Goliath actually came after Godzilla vs. Redmoon was considered by Toho, and was reportedly the result of major alterations to its script. Popular images of Redmoon and Erabus on the internet are actually just photoshopped images of Goliath and Daigoro's Mother from Daigoro vs. Goliath. The most reliable source towards any description of the three monsters' appearances is a piece of concept art drawn by Hurricane Ryu Hariken years after the concept was scrapped.