While using a texture pack I used TextureEnder on, (It makes converts texture packs be compatible with MC versions that don't support texture pack coding style) the Zombie and Zombie Pigman texture files are flipped, with some parts of the texture completely missing.
I attached the texture files to this thread so if anybody wants to inspect them and tell me the problem, they can.
With that update, those textures became perfectly square files rather than rectangular. Increase the canvas height of the image (do NOT use the "Resize" option) to equal the width, and then move the existing texture to align with the top of the image. Depending on your software, there may even be an "align to top" option right in the Canvas Size options box which will save you a step.
I made the textures taller using "Paint" so that it would be a perfect square, but one problem...
The heads of the zombies have a second layer on their texture, making their head be covered with pure white! The zombie pigmen are fine, the zombie is my only problem.
Could you please get rid of the white on the texture for me? I don't know how to.
Don't use Paint. It doesn't support transparency. GIMP and Paint.net are both freeware, and they both support transparency. There are loads of other freeware programs out there as well, if neither of these will work for you for whatever reason.