Executive Meddling: Ishiro Honda originally wanted to make the film without a giant monster, but Tomoyuki Tanaka insisted that there be one. Thus, the Moguera was born. Not that this is a bad thing, mind you.
Also the reason the RKO version was not present on the DVD. Toho hates this version for no reason, and the copyright should've reverted back to them in any case.
Hey, It's That Voice!: Bret Morrison, who dubbed Kumayama in the English dub of Mothra vs. Godzilla, voices several characters in the original English release, possibly the Mysterian leader as well. Many other Titra/Titan actors are present, though their voices are harder to recognize because they were all younger and didn't put on the phony Japanese accents for the most part.