I actually do disagree. It's DARKrai. Also, it's not based on a ghost, it's based on the shadows, nightmares, and potentially Morpheus, none of which have anything to do with ghosts.
The ghost Pokemon in Pokemon lore are more related to shadows and nightmares, more so than the Dark type pokemon. Dark type Pokemon are all about dirty fighting and trickery. The Ghost type pokemon attack using "darkness" with moves such as Shadow Sneak (their shadow attacks the opponent) and Nightmare (they haunt the opponent in their sleep).