Ghosts (Heretic)

Some monsters in Heretic (Golems and Undead Warriors) can appear in a form of a ghost. Ghosts are translucent and cannot be harmed by certain weapons, which are defined as non-magical weapons.

Additionally, the Disciple of D'Sparil seems to possess the ability to flicker in and out of tangibility when preparing its attack, so a player might find a couple of their crossbow bolts going right through them, for example.