In an interview with "Godzilla vs. Space Godzilla" director Kensho Yamashita, he reveals two monster concepts that were explored in preproduction, but never ended up in the final version. The first of which was a brand new G-Force built Mechagodzilla, since the original one was totally destroyed in the previous year's film "Godzilla vs Mechagodzilla II" and was to take part in battle along with Godzilla against his hideous blue alien doppelganger. But Koichi Kawakita, the director of special effects, made the decision to use MOGEURA, an update of the robot from 1957's "The Mysterians" instead. According to Mr. Yamashita, this allowed the battles in the film to take place in a wider variety of settings (MOGEURA is formed when two different vehicles, the Star Falcon, which can fly in outer space, and the Land Mogera, which can tunnel underground, are combined).

The second abandoned monster concept was an idea suggested by producer Shogo Tomiyama, in which three giant dragonflies would menace Little Godzilla on Birth Island. But Mr. Yamashita admits that he (and probably others) decided not to use Tomiyama's idea. Nothing else is known about these dragonflies outside the above mentioned.

Entry written by Rafael C. Gonzalez
Source: an interview with Kensho Yamashita, conducted by David Milner in December of 1994 and translated by Yoshihiko Shibata.