Darth Plagueis was named as early as the first draft of Revenge of the Sith (April 2003), and possibly earlier than this. His name is clearly derived from the English word "plague," a connection later made in-universe in the Wizards of the Coast article Behind the Threat: The Sith, Part 2: The Becoming.
A non-canonical image of Darth Plagueis from Tag & Bink: Revenge of the Clone Menace.
Plagueis' first appearance of any sort was in the non-canon Tag & Bink: Revenge of the Clone Menace. In this comic it was stated that Plagueis' manipulation of the midi-chlorians produced Force-sensitivity in Tag Greenley and Bink Otauna. It also caused the destruction of a red sun and its planet, whose only survivor would go on to become a hero on Earth—references to Krypton and Superman, respectively.