This is a theory that is very popular and has been passed on the web many times!

As you see, a shuppet might be a lonely Rotom who one day came across a reaper cloth! Their height, weight and shape are identical in the games!

It was so lonely that it wanted to not and never be recognised again! It flew to the reaper clothed and hid under it, thus forming shuppet!

 Shuppet is a ghost type, ghost types are usually very hard to train and hide in the shadows! Think about it, could Rotom have gained the ability to hide in shadows in order to avoid others? Could this be why ghost types are said to be hard to train?

Also, one of the Rotom forms is a ghost type! Could this be linked? 

They are also both stated to 'levitate'.