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.
The simplest would be to just create a material with alpha transparency and alpha set to e.g. 0.5 to be semi transparent. Apply the material to some object (cube/plane/etc.) in the scene and anything behind this object should appear as if behind a tinted glass. Or if you want something more sophisticated, then someone already posted links to ice shaders you might want to look into.
Is this the effect you're after?
https://piped.video/watch?v=uLN69rtiu4Y
It's definitely possible with custom shaders, thought there might be somewhat of a learning curve.
If it was in 2D, you could couple the sprites/textures under a CanvasGroup node.
In 3D, I suspect you'll want to apply a NoiseTexture3D. Although this video uses the 2D noise, I suspect it won't be too different in how to make it work for a 3D texture - https://youtu.be/IvOfx-kbqac
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