This is an Event Pokemon released for the 2010 Halloween Event. It was only available for one day, the 31st of October, and came holding a Pumpkin Powder, which could be used for another part of the event.

When you put GHOST into your party, the rest of your party will turn into mist.Party

It doesn't matter which slot you put it in. Aditionally, GHOST will cause your Trainer card to have a fainter trainer image and a little cross next to your name.

TRainer card