Some Godzilla Stuff that almost existed in the 50s and/or 60s and/or 70s

http://www.kaijuphile.com/rodansroost/scrapyard/rogshowa.shtml

 https://www.kaijubattle.net/kaiju-book-database/toho-special-effects-official-visual-book-vol20-king-ghidorah-1964

From Kaijuphile:

Return of King Ghidorah

Screenplay written by Shinichi Sekizawa

Alein invaders plan to scour the earth of its despoling population of humans before the world can be polluted beyond repair and becomes useless for their plans of colonization. Towards this end, the aliens bring the monster Mogu, Gigan and King Ghidorah to destroy human civilization. But their arrival does not go unopposed. The monsters Godzilla, Rodan and Varan escape from Monster Island to confront the alien space monsters.

History:

After his bad reaction to Godzilla vs the Smog Monster, Toho producer Tomoyuki Tanaka ordered that a new script be produced for the next Godzilla movie, this time featuring his classic foe, King Ghidorah. The Return of King Ghidorah was one of two scripts submitted and contained many of the elements that would later become Godzilla on Monster Island.

So this weird monster flying in the middle must be Mogu, who I think never showed up for real in any Godzilla movies. Only in this script. ???

Next is the stuff from Kaijubattle.net ... 

Toho Special Effects Official Visual Book Vol.20 King Ghidorah 1964
Publisher: Dai Nippon
August 2018
​Pages: 36
Planning and organization: Osamu Kishikawa
Designed by Makena Graphics
​Language: Japanese & English​​

Synopsis
All volumes are published by Dai Nippon Printing, read from left to right, each volume is 36 pages long and printed on A4 size paper. They are distributed by the Godzilla Store and cover the conceptualization and production aspects of kaiju from Toho's library of films, with each volume focusing on a different character.

This volume covers King Ghidorah from Ghidorah The Three-Headed Monster (1964).