King Kong vs. Frankenstein was a project originally conceived as a sequel to the 1933 film King Kong, with a treatment written by animator Willis O'Brien featuring King Kong battling a large monster created by Frankenstein's grandson in San Francisco. John Beck (an independent producer) stole the story from Willis O'Brien and, since he could not find an American buyer for the story (which was fleshed out into a screenplay by George Worthing Yates and retitled King Kong vs. Prometheus), sold it to Toho Studios who replaced the Frankenstein/Prometheus monster with their own monster Godzilla, and filmed it as King Kong vs. Godzilla instead.
Toho had always been interested in the Frankenstein character, as years earlier they had planned to film a movie called Frankenstein vs. the Human Vapor (フランケンシュタイン対ガス人間?, Furankenshutain tai Gasu Ningen, lit. Frankenstein vs. the Gas Human), a follow up to their 1960 film The Human Vapor, that fell by the wayside. Influenced by the concept of the giant Frankenstein monster from the King Kong vs. Frankenstein/King Kong vs. Prometheus story, Toho planned on making Frankenstein vs. Godzilla as a follow up to King Kong vs. Godzilla. Written in 1963 and planned for a 1964 release, the story dealt with the heart of the original Frankenstein monster becoming irradiated and growing into a Frankenstein-monster giant. Afraid the giant would start eating people, Godzilla would be freed from an icy prison by the JSDF and goaded into a fight with the monster in hopes of killing him. Even though King Kong vs. Godzilla had already been made with Godzilla escaping from an iceberg that he was trapped in at the end of Godzilla Raids Again, script writer Kaoru Mabuchi (a.k.a. Takeshi Kimura) thought with Godzilla disappearing into the ocean at the end of that film, that the idea of Godzilla becoming frozen in the North Sea into another icy prison could still work. The story would end with Godzilla disappearing into a raging river flow, while the Frankenstein giant disappears into magma.
Toho thought the story would not make any sense because the JSDF are trying to get Godzilla (who was still a villain at this point) to kill Frankenstein because they are afraid Frankenstein will start eating humans. So the idea was dropped and Mothra was brought in as Godzilla next opponent for the film Mothra vs. Godzilla. Toho didn't give up on the Frankenstein giant idea. In 1965, the monster would appear battling a new monster opponent named Baragon in the film Frankenstein vs. Baragon, which used a lot of concept ideas as well as the same characters, such as Dr. Bowen, from this story treatment.