Erabus and Redmoon are both members of a species of giant creatures whose male and female members display an extreme case of sexual dimorphism. Their exact origins are ambiguous, as Redmoon first appeared from the moon in outer space while Erabus surfaced on Earth.
Erabus came to Earth shortly after Redmoon arrives from the moon. The Japanese Self Defense thought they could lure the two kaiju together so they could defeat each other, but they turn out to be the same species and of opposite genders. They soon have a son dubbed Hafun, who is killed later on. Hafun's parents go berserk and destroy cities, but Godzilla arrives and defeats them.