https://yokai.com/sazaeoni/
https://hyakumonogatari.com/2013/06/25/sazae-oni-the-turban-shell-demon/

I always thought Shellmon was based on this female testicle stealing oni so this isnt really a surprise and also fucking mermaids are the most common shit ever and she seems to have healing properties from the yokai amiebie. During Covid people would post it for good luck because it can proohecize disease and cure shit if you spread its image.

Slowbro from Pokemon was probably based on the same shit and and Amiebe was in SMTVV

The shell on her head makes it seem like the final evolution was inspired by it too