Ghost Monsters
 

It is possible to create a semi-transparent image. With such in mind one could easily make a semi-transparent body for any monster and make a ghost/undead version. This would fit the haunted houses and bring some new content to the over-used haunted houses.

The way that it seems to work is that you'd have to have a script to make it transparent. Or you could remove have of the pixels in an image to make it look transparent. GK already has a ghost image for prospect players, I say why not use that image?