There are supposedly 2 ways SpaceGodzilla was created. Traditionally, we see Mothra make appearances, along with her twin fairies (voices anyway), and when she flew into space after Battra was dropped with Godzilla, she had Godzilla cells on her, and they apparently were sucked and mutated in a black hole. The other possible theory was that the cells came from Biollante, but the Mothra way is more leaning.
My YouTube Channel - [link]
SpaceGodzilla Art Video - [link]
To see the full English dub movie of Godzilla vs. SpaceGodzilla, please click here -> [link]