Ghost (monster class)
All ghosts are humanoid, human size undead monsters that are unsolid and breathless. They are capable of flight and phasing through walls, and will follow a nearby player to other levels. They cannot be genocided, and are always generated hostile.
Some variants of NetHack change the glyph used by ghosts for better visibility.
SLASH'EM adds one new monster to the ghost monster class:
In xNetHack, this monster class is merged into the wraith monster class to avoid using punctuation for monster glyphs and make them more visible.
SpliceHack adds a new monster to the ghost monster class:
Regular ghosts can grow up into spectres.
EvilHack also adds spectres to the ghost monster class.
In SlashTHEM, the ghost monster class now uses the 8 glyph. SlashTHEM retains the shadow from SLASH'EM and adds new monsters: