ChickenShades

ChickenShades is a client-side only mod which draws an extra layer of graphics onto entities. By itself it does nothing, but when supplied with a texture pack with images in assets/chickenshades of the format [entity registry name].[visible name].png it will draw them on as an additional skin layer.

The titular example is Vaygrim's chicken shades, which take the format of chicken.vaygrim.png, or when using a chicken from the Chickens mod, chickens.chickenschicken.vaygrim.png (what a mouthful!)