Clados Vader

Kurado Vader was a mysterious changeling entity who dwelled in the oceans of Iskalon. It was a giant deformed shark with a pinkish hue, and it could turn into the shape of the Sith Lord Darth Vader – hence the creature's name.

In 0 BBY, Kurado Vader held C-3PO prisoner in an underwater chamber. Luke Skywalker came to rescue the droid and confronted Kurado Vader ; one swipe of the Jedi apprentice's lightsaber and the fake Vader transformed back into its real shark-like form. After Kurado Vader was defeated, Luke Skywalker and C-3PO set off for the Death Star.

Behind the scenes
Kurado 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 canonicty of this game and the creatures it features is dubious.