I'm trying to change the skin of a mob example "Zombie Pigman" and i do not understand how the mobs.json is setup for the mob as far as the x,y,z / Pivot and UV's work. Can someone who knows how to do this educate me in learning how and what to look for to get a better understanding of how these textures of mobs work?
If you are just trying to change the skin and not trying to completely adjust the model, I'd just look at the default skin and edit it directly. if you want to make a higher quality skin just resize the original skin in the vanilla resource pack (textures\entity\pig\pigzombie.png) to your new size (maintain proportions and when resizing use nearest neighbor blending) and then adjust the texture that way.
If you are looking to completely adjust the model to make it look like a different animal I'm sorry I can't help I haven't gotten that far into the minecraft resource editing, just been doing re-textures.