There have been a couple of posts on this.

One excerpt I believe somewhat answers your question from the same thread, answered by /u/batmaaaaaaaaaaan

But Italy at the time of Rome's founding was dominated by the Etruscans, Samnites, and other civilizations, which means it's likely that early Romans, if they weren't following their own versions of Greek gods already, may have practiced some Italian religions with different sets of gods altogether from the ones we associate with Rome.  

So what where the myths about Jupiter before he became just another name for Zeus? There are three options. His myths were lost to time. He and other early Roman gods were gods without any myths (a popular theory today supported by what we know about Roman religious, cultural, and political practices). Or he was always Zeus by another name (like historians wrote during Roman times).