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.
Not to get too technical, but the name Palpatine was actually first used in the 90’s EU.
No, it was first used in the prologue of the 1976 novelization of Star Wars. So technically the name Palpatine predates any of the films, even if it wasn’t used in a film until 1999.