I'm not terribly proficient with making materials yet, but I was wondering if something like this is doable and if so, what it might involve. I basically want to make a part of a creature appear beneath the ice in 3d. but I don't know how to approach it or what I should be looking at.
Check out an ice shader (ex: https://godotshaders.com/shader/spatial-ice-shader/). I'd imagine you could make a rectangle or plane with a material using the shader and place it over your creature. There's lots of good information in the Godot docs on shaders, I would recommend taking a look once you have a good grasp on the basics with materials.