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There are two assets folders in minecraft, you want to make a copy of the one in the minecraft jar. Edit those, and then in order to turn it into a resource pack you have to get something called mcmeta to out in your zip file with assets. To get mcmeta you can get it from copying it out of another resourcepack, then editing what it says inside of the mcmeta to make a pack description.

just copy your assets folder, the mobs are in assets < minecraft < textures < entity < [mobname] sorry if i read the question wrong