Feature Request: procedure to change entity textures so we can create variants

Horses, rabbits, cats, parrots, tropical fish, sheep, etc. as existing vanilla examples. Please also add the ability to add texture layers, like how horses have color bases with patterns that overlay them, and like how horse armor overlays the horse texture as well. Adding this would do wonders for the community by expanding their creative capabilities and variety in their mods. I've seen lots of people searching for how to achieve this.