Sasori Vader

Sasori Vader was a changeling entity who lived on planet Tatooine at the time of the Galactic Civil War. Its true form was that of a giant scorpion with red glowing eyes, but it could turn into the shape of the Sith Lord Darth Vader.

In 0 BBY, Sasori Vader sent his Jawa minions to kidnap R2-D2 and imprison him in the basement of their sandcrawler. The young Luke Skywalker then made his way to the sandcrawler in order to free his droid friend and confront Sasori Vader, who had taken the shape of Darth Vader. During the ensuing fight, Sasori Vader turned back int its true form, but he was eventually defeated and killed by Skywalker.

Behind the scenes
Sasori Vader was one of the stage bosses in the Japanese Star Wars 1987 video game. The game featured various fake Vaders which all happened to be shapeshifting monsters. The canonicity of this game and the creatures it features is dubious.