Ghoul

A ghoul is summoned when you engrave on a headstone. Another ghoul will not be generated when you engrave on the same headstone again.

There was a bug where one can repeatedly generate ghouls by engraving on the same headstone over and over again (with no limit). This bug was fixed in NetHack 3.4.0.

Encyclopedia entry
The forces of the gloom know each other, and are strangely balanced by each other. Teeth and claws fear what they cannot grasp. Blood-drinking bestiality, voracious appetites, hunger in search of prey, the armed instincts of nails and jaws which have for source and aim the belly, glare and smell out uneasily the impassive spectral forms straying beneath a shroud, erect in its vague and shuddering robe, and which seem to them to live with a dead and terrible life. These brutalities, which are only matter, entertain a confused fear of having to deal with the immense obscurity condensed into an unknown being. A black figure barring the way stops the wild beast short. That which emerges from the cemetery intimidates and disconcerts that which emerges from the cave; the ferocious fear the sinister; wolves recoil when they encounter a ghoul. [ Les Miserables, by Victor Hugo ]