Kaiju no 14 discovered something
Apparantly the All Toho Monsters Pictorial Book says
未制作映画『ネッシー』の原型が流用された。
This roughly translates to "The prototype of the unproduced movie "Nessie" was diverted"
https://wikizilla.org/w/images/c/c1/Nessie-Unmade-Ckf16oJWgAAH0EA.jpg heres the image in question as well i think
If im understanding this correct it seems to say the Dragon was based on a model made for nessie not a reused prop