In the case of creating a zombie in the likeness of a famous zombie from a movie, I would say that as long as you don't try to sell the asset but just make it available for free use then you probably won't run afoul of any movie studio. The issues arise the moment you start trying to profit off of the IP of someone else. Since mods are free there really is no action. If TFP wanted to use that famous zombie in their game then they would have to obtain a license and pay royalties because they would be using that IP to help sell their game.