In the original concept King Ghidorah was colored purple and his wings were rainbow colored. Also, the creature was to breath fire from it's three heads. In the final concept they emit lightning-like gravity rays and his skin is colored gold. Various publicity stills and promo material however still show parts of him in their original colors. On the movie's poster, only his body is colored gold, because the suit had not been finished when the shot was taken.
When Godzilla first rises from the ocean, the costume appears to be the complete suit used in the prior film (Godzilla vs Mothra) which features a longer and more reptilian looking head. This does not match the head used for the monster throughout the rest of the film. Toho commonly used older versions of their monster suits when filming scenes that take place in water, in order to avoid wear and tear on the new featured costume.
The movie was for a long time unknown in European countries that otherwise used to screen Japanese monster movies, because for whatever reason, it was only released in a select few areas, such as Turkey, and only came to many of the other countries when the DVD was made available.
Because of its commercial success from King Kong vs. Godzilla, Toho decided to release another Godzilla film in 1964, Ghidrah, the Three Headed Monster. The Mosugoji suit, though, needed to go through some repairs. During the filming of Godzilla vs. Mothra, the mouth had become wobbly and would not work properly. The head had also caught on fire, which didn't help the suit much. So the head of the Mosugoji suit was replaced by a more sturdier design. "Mosugoji II," as some fans refer to the repaired suit as, also had more mobility than the original Mosugoji.
In this sequel to "Mosura tai Gojira" (1964) Hiroshi Koisumi returns as Professor Miura. Yuriko Hoshi also returns as the female lead but she plays Naoko, a different character than she played in the previous film.
Rodan was originally introduced in the movie Rodan (1956), and would become a recurring character in the Godzilla films later on. It is never explained whether he is one of the original Radons (who apparently were killed), or a new specimen, although it is likely the latter.