Tohl Narita: “This is a design being Godzilla with a muffler scarf attached to it.”
Such stuff around the neck like a frill-necked lizard is called “erimaki” signifying “muffler scarf” as the lizard is called “erimaki tokage” in the Japanese language, so I followed suit in translating Narita’s description of Jirass (Jirāsu in Japanese pronunciaton).
When looking at Narita’s design drawing, it makes me feel like it shows his unwillingness to design a kaiju based on the existing Toho kaiju as the monster drawn by him looks more like a dinosaur than Godzilla while Godzilla’s characteristic fins on the back are drawn with more of a Narita style (I personally think the design is worth being sculpted into another original kaiju just as it is).
To avoid any misunderstanding, needless to say, it was not Narita who decided to use the Godzilla costume for Jirass but producers made the decision probably to reduce cost and to adjust the filming schedule.
It is said that the Godzilla costume used for Jirass was the one with the head from the costume used in the Toho movie “Invasion of Astro-Monster” (1965) and the body from “Mothra vs. Godzilla” (1964) put together.
This combined costume seems to have been displayed at a supermarket called Akafudadō in Ueno back then before being lent to Tsuburaya Productions while it is undescribed why such a combination was done (it is thinkable the other costumes should have been in an unusable state, of course).
The frill was sculpted by Akira Sasaki who says he remembers the Godzilla costume was brought in from the Toho Studio and that he must have used bamboo to hold up the frill made of latex from within while he also recalls he attached the frill by pasting rubber onto the neck and the end of the frill.
He says it made the remodeling even harder as the costume had to be returned to Toho intact and the frill had to be placed firmly while it was to be removed by Ultraman in the end.