Guiron

Guiron was designed by Tomohisa Yano on behalf of Akira Inoue, who suffered a backache while working on preparations for the 1970 World's Fair before work began on Gamera vs. Guiron. According to director Noriaki Yuasa, Yano initially wanted the new monster to be a flounder-like monster with the actor entering its suit sideways, but this was too difficult to realize. Inoue commented that the design of the enemy monster was "biologically in mind" at first, and of the final design of Guiron - which was far from "biological" and contrary to this idea - said "I created an amazing monster anyway. The staff came up with the idea of having the whole body be a weapon, and when I put the head of a knife on a body I created Guiron. It's not a living thing anymore, the idea comes from a weapon. Only Guiron is like this." Looking back on the series, Inoue cited Gamera vs. Guiron as his favorite work.

While Ex Productions was typically in charge of modeling monsters for the Gamera series, the company was busy modeling Yongary for the South Korean monster film Yongary, Monster from the Deep. Instead, Eizo Kaimai's Kaimai Productions handled modeling of the Guiron suit. An elaborate miniature model of Guiron was also created.

Guiron was revisited as a possible foe for Gamera in the second entry of the Heisei trilogy, but did not make the cut. However, many features of Guiron such as his extraterrestrial origin, facial horn used as a weapon, and a body made of inorganic material were present in the Mother Legion. Director Shusuke Kaneko stated that he was in charge of writing the script and coming up with ideas for Gamera 2 with Shinji Higuchi and said that Guiron was indeed considered as an enemy monster. Writer Kazunori Ito stressed, however, that Guiron did not form the basis for the Mother Legion, who was conceived based on ideas for the story. Ito did however state that Legion being a space monster may have been influenced by Guiron.