So this is really interesting.

It seems Godzilla existed within Mothra's canon from the very beginning. The mention of multiple godzillas is also curious.

Does it imply an active population, or just the first and second generation Godzilla that appeared in the Showa era?

For anyone curious, this is from "The luminous fairies and Mothra," the book Toho commissioned to serve as the outline for her very first film.