It is not just the model, but also the idea behind it and its design which is copyrighted. Lets say that you create the perfect predator game and publish it for free. It for sure won´t be possible anymore to sell the ones which really have the rights to do so. So there you could cause some financial damage or whatever. It doesn´t matter if your project is free or not, point is that you don´t have the rights to do so without asking. Some don´t care, others care a lot and shut any project like this down.
Best is to just ask and if you are not allowed to do it, create something similar but based on own ideas and designs.