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There's a big difference between copyright and a patent. I'm sure you meant copyright. It all depends on the license for the particular model in question. Typically, the "general" royalty-free licensing allows for the use of models in commercial games but also retains the intellectual property of the rights and ownership of that model to the original artist/author. In other words, you should probably ask the artist for permission because they retain the rights to the model in the end.
So the bottom line is that, for any commercial project, it's always best to not only review the license associated with that product but also contact the owner and ask them if it's ok, even on royalty-free, general licensing. Good luck!
