Since no human being could have thought up a name like Quetzalcoatl, he must have been real, right?

Think about it. If you're going to make up a deity, you're going to give it a name that rolls off the tongue, otherwise people aren't going to put in the effort to worship it. Zeus. Osiris. Jesus. Allah. Short, pithy, easy to say. So clearly this name indicates that it wasn't made up by man, right? It must have been the feathered serpent's real, given name, so he must have existed to tell it to some lucky Aztec.