It’s a name that was created for that storyline. Palpatine, much like the term Sith, was used for the OT and its marketing materials. Sidious was treated as his truer name, with Palpatine being more of the disguise.
Every other film in the franchise, including first instalments of two other trilogies. This wasn’t like Lord of the Rings, where it’s one story spread over 6-9hrs. It was a first story, but he failed to tell a complete story.