# Extinction
In NetHack, extinction is the condition whereby a particular kind of monster can no longer be (normally) generated after 120 of its kind have been generated. Once a species has become extinct, there are very few ways to create new monsters of that species. There are some exceptions, however, such as the monster spell summon nasties. Extinction is distinct from genocide, which must be deliberately invoked and has no exceptions.
Extinctionism is the challenge themed around rendering every possible monster in a game of NetHack extinct. 

A user has suggested improving this page or section as follows:
"Add source code references, describe mechanics more precisely than "actions that sometimes respect extinction".  What actions increment the born counter and respect extinction?  What actions increment the born counter but don't respect extinction?  What actions respect extinction but don't increment the born counter?  Also, does unstoning a monster actually contribute to extinction in the current version?"

Contents

1 How creatures become extinct
2 Actions that respect extinction
3 Actions that sometimes respect extinction
4 Actions that do not respect extinction
5 Monsters not subject to extinction
6 Play styles for an extinctionist
7 Special cases
8 References
9 External references

How creatures become extinct
A creature is extinct when 120 of its kind are generated.  The creatures do not have to be killed to be extinct.  Once extinct, a creature will never be randomly generated again.
For example, you can make a creature extinct simply by summoning, or reverse-genociding 120 of its kind (which can be expected to take approximately 24 cursed genocide scrolls, given that each scroll summons 4-6 individuals), with no requirement to kill the summoned creatures.
As exceptions to the normal rule, erinyes become extinct after three are generated, and Nazgul after nine are generated. Extinction of these species is more thorough; even methods that ignore or only partially respect it will not generate them.
To make a creature extinct, you must create it in a way that increments the "born" counter.
Mechanisms of monster creation can be classified in those that

respect extinction and increment the born counter (examples: normal random generation; create monster spells; etc)
respect extinction but do not increment the born counter (example: pudding division)
do not respect extinction, but do increment the born counter (examples: Summon Nasties; Quest-specific monster generation in the Quest)
neither respect extinction nor increment the born counter (example: troll regeneration).
Actions that respect extinction
Reverse-genociding a monster.
Casting create monster via spell, scroll or wand.
Waiting around on a level for it to be randomly generated.
Entering a level containing a special room such as a zoo, or beehive.
Black puddings summoned by kicking a sink[1].
Division of puddings and gremlins
Applying a bag of tricks
Stepping on a magic trap
Special levels which place monsters of a specific monster class.
Pudding and gremlin division does not increment the born counter but respects extinction. You cannot make puddings extinct by dividing them, but once they have been made extinct by other means, puddings will no longer divide.[2]

Actions that sometimes respect extinction
Throne rooms, and sitting on/confusedly looting thrones.
Bugbears and hobgoblins may be created even if extinct.
Graveyards and reading the cursed Book of the Dead.
Master liches, nalfeshnees, ghosts, and wraiths can always be generated.
Antholes.
If every species of ant is extinct, ants will be replaced by random monsters which can include extinct species.
Shrieker summoning.
Purple worms will not respect extinction if explicitly rolled. If they are selected as part of a random monster roll, on the other hand, they will respect extinction.
Demon summoning
Only unique demons will respect extinction when generated by this method.
Spider webs
Spider webs generated as dungeon features will not respect extinction, and will always have a spider under them.
Actions that do not respect extinction
Bees in a beehive (queen bees, killer bees)
Cockatrice nests
Leprechaun halls
Summon nasties monster spell
Troll regeneration (does not increment the born counter)
Scroll of create monster while confused (acid blobs)
Keystone Kops summoned when you've stolen from a shop[3]
Quest themed monsters when replacing ordinary random monster generation in that dungeon branch
Golems created from polypiling
Figurines
Special levels which place monsters of a specific species.
Monsters not subject to extinction
Because the most common methods of extinctionism rely on monster generation, unique monsters are ignored by some extinctionists. However, unstoning a statue of a monster will increase the monster count for purposes of extinction; as a result, even unique monsters can be made extinct in this sense. Note that unlike unstoning a monster, reviving a corpse does not increment the born count of the monster; since extinction is based on the number of monsters 'born' rather than the number killed, the method of revival rather than the method of killing is the determining factor.
Keystone Kops do not become extinct even after 120 of each kind have been created.

Play styles for an extinctionist
 Extinctionist wizard has filled an entire level with monsters by repeatedly casting create monster.
The easiest way to play an extinctionist is to play as a wizard.  Wizards have the ability to cast create monster with no hunger penalty, and with the Eye of the Aethiopica they can cast virtually indefinitely.  There is no reason another class couldn't do extinctionism; it would simply be more challenging.
An alternative to the Eye is confused throne looting. Once extinct, the typical throne room population is replaced by ordinary random monster creation. You can generate as many monsters as you like, for free, at any rate you like, without even breaking conducts. However, killing the monsters is slower due to confusion, so several strong pets might be a good plan. The drawback is pet kills will not generate death drops.
Most extinctionists play through the early- to mid-game as normal, then start working on eliminating monster species after becoming strong enough to do so safely. "Strong enough" in this case typically means obtaining the Eye, sporting a decent set of armor and a good weapon, and gathering up all the various resistances, including magic resistance, and reflection.  It would be very bad to lose your character to a random gnome with a wand of death after hours of painstaking and tedious summoning.
A common method to rack up kills is to build a boulder fort around an altar.  Then stand behind the altar and cast create monster over and over, while killing the resulting monsters and sacrificing them to your god.
You can either just summon monsters and kill them at random, or try to control your summoning.
Controlling your summoning can be done by carefully regulating your own experience level (either by foregoing experience gain by letting pets and/or conflict take care of kills, or by casting drain life on yourself when required), and picking which dungeon level to summon the monsters on.  By keeping your experience level low and staying on a low-numbered dungeon level, you force the game to only summon monsters of a particular difficulty or lower.  In this way you can wipe out all the lower-level monsters, casting and killing until create monster fails.  You can then move down one or two dungeon levels, gain one or two experience levels, OR move down one dungeon level and gain one experience level -- any of those effects will increase the difficulty of available monsters by one (since monsters are generated of difficulty not exceeding to the average of your dungeon level and experience level).
This controlled summoning method could also be used from the beginning of the game, before acquiring the quest artifact and key resistances, although as noted above this method is somewhat more fraught with danger.

Special cases
Erinyes become extinct after 3 have been created and Nazgul become extinct after 9 have been created, as would be expected based on their respective sources (although not all authors held the Erinyes to number three).

References

↑ dokick.c in NetHack 3.4.3, line 956

↑ makemon.c in NetHack 3.4.3, line 679

↑ There is code to make them extinct shk.c, line 3435, but wizard mode testing confirms you can have as many Kops as you like.  This is because propagate in makemon.c never marks non-randomly-generated (G_NOGEN) monsters such as Kops as extinct.

External references
extinctionism spoiler