King Kong's creator, famed animator Willis O'Brien, conceived a script (in the 1960s) that he called "King Kong vs Frankenstein", planned second "sequel" to his original 1933 film. It was never made, though in the end, as "King Kong vs Prometheus", it was sold to Toho Studios in Japan, who re-conceived the project as Kingu Kongu tai Gojira [King Kong vs Godzilla] (1962; dir. Ishiro Honda) -- and then again, sans Kong, as Furankenshutain tai chitei kaiju Baragon [Frankenstein vs the Subterranean Monster Baragon] (1965; dir. Ishiro Honda) aka Frankenstein Conquers the World (US, 1966).
Below ate two conceptual sketches O'Brien made for the film.