A/N

In Prisoner of Azkaban, the scene when Snape caught Harry with the Marauder's Map, he tried to use the revealing spell on the map to make it show its secrets. Nothing happened. Or... did it?
Ever since that incident, the map had a new name inscribed on it along with Wormtail, Moony, Padfoot, and Prongs: Sharpclaw. Harry never noticed until Hermione pointed it out to him many years later.
Why was this fifth Marauder not mentioned with the original four? What happened to this 'Ghost Marauder?'

Were they forgotten on purpose?