Ghost monster

Ghost monsters are caused by a rare bug that occurs when a monster's corpse is crushed under a door or ceiling and then resurrected by an Arch-Vile. These ghosts behave like the monsters they formerly were, except that they can sometimes pass through walls and can only be harmed by splash damage or monster's melee attacks.

This bug is fixed in nearly all source ports. However, some PWADs have used the ghost monster phenomenon as a feature. Boom and its closer derivatives can be set to allow or disallow the resurrection of ghost monsters according to the user's preferences.

Ghost FAQ at gamefaqs.com