Dungeon fern

The dungeon fern an unassuming UnNetHack monster. It is sessile and has no physical attacks. However, when approached, it creates dungeon fern spores, the more dangerous cousins of gas spores.

Dungeon fern spores attack the player directly by bursting into stinking clouds. This effect is harmless, and the dangerous effects of the cloud are entirely mitigated by poison resistance or unbreathing. If you do not have either of these, standing at a diagonal to the spore when it bursts will give you a significant chance of being outside of the cloud when it forms. Attacking directly will cause the spore to explode properly, just like a gas spore.

Exploding dungeon fern spores have a chance of creating dungeon fern sprouts, lower level dungeon ferns which produce spores at a lower rate. Full grown dungeon ferns will still be produced from time to time, however. It is important to destroy both kinds of ferns quickly unless you possess the means to contain out-of-control growth.

Other monsters and dungeon fern spores despise each other, and both will attack each other on sight. A careful player can take advantage of this and lure spores toward otherwise difficult monsters without poison resistance.

Encyclopedia entry

These dreadful weeds have been driven to extinction on the
surface, yet deep within the dark and damp of the dungeon
they still flourish. When it detects the motion of nearby
creatures, the dungeon fern releases a deadly, poisonous
spore, which quickly detonates to propagate the species.

Residents of the dungeon have learned how not to disturb
the dungeon fern, and will attack its spores on sight to
keep them from overtaking their home.