@Whonk
Sadly, that turned out to be Invisa-sunset. Who, being slightly Evil like Sunset was before, has used said invisibility to become the greatest thief Equestria has ever known. They call her: The Phantom Sunset.
Clearly, it is the fate of all Sunsets to move to another dimension. Human Sunset went to another universe, where everyone is, say, a shark, and she took the place of Sharkset, Sharkset then took the place of Dinosaur Sunset, and Dinosaur Sunset replaced Robot Sunset, until eventually Pony Sunset was replaced by whichever Sunset closed the loop.