I was curious about what shader you’re looking for (I’ve never played a pokemon game tbh 
), and I found this reference:
https://youtu.be/bwNmp6RoWUs?t=333
Are you trying to recreate the shader on the belly/bottom jaw area?
A simple version would be a color gradient mapped to fresnel.
If you want more details on the inside then maybe parallax offset, or a second mesh inside with flipped normals? “galaxy” shaders like this one have a similar styling.