I’ve long been familiar with this sketch by Willis O’Brien (famed animator of the 1933 King Kong):
But I only stumbled upon this one yesterday while looking for something else:
These are both conceptual sketches by Willis O’Brien. O’Brien conceived a script (in the 1960s) that he called “King Kong vs Frankenstein”, planned as a 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).
Of course, O’Brien felt betrayed by what Toho did to his script, though I believe he died before actually seeing King Kong vs Godzilla itself. If he’d had a chance to see the original Japanese version (as distinct from the appalling US re-edit), maybe he would have gotten the joke … I doubt it, though. O’Brien was committed to stop-motion SFX and his “King Kong vs Frankenstein” would have had both monsters animated using this technique, rather than the trademark suitmation of Toho Studios. (Interestingly, director Ishiro Honda was considering using stop-motion for his “King Kong vs Godzilla”, but it would have been too expensive for Toho’s available infra-structure.)
Oh well. That’s another great Willis O’Brien project that we’ll never see; compare the Backbrain War Eagles article.