The K Keystone Kop monster class comprises human law enforcers that are summoned whenever you steal from a shop, such as by teleporting out of the shop or digging downwards. Outside of those situations, the only time the Keystone Kops are otherwise seen is during the Tourist quest, where several of them occupy the police station on the last floor.
Keystone Kops attack you by throwing cream pies and using the clubs and rubber hoses they bring with them. While they are each not very difficult to eliminate on their own, their numbers are quite intimidating. Fortunately, Kops generated in response to theft will vanish as soon as you pay or pacify the shopkeeper, together with anything they pick up. Watch your missile weapons.
There are four kinds of Kops: Keystone Kops, Kop Sergeants, Kop Lieutenants and Kop Kaptains. Keystone Kops will not become extinct after 120 of each kind have been created, though they are genocidable.
The Kops are a very good source of experience and cream pies.
One minor strategy with shopkeepers is to steal a gold piece from the shop and leave for a distant corner of the dungeon; this will summon a slew of Kops and their equipment, followed by the shopkeeper. After slaughtering the Kops, pay or pacify the shopkeeper. Note that Kops that evaporate in this way don't leave death drops. Warning! If you angered him by something else than stealing, it will take at least 1000 gold to pacify him; make sure you have that much cash.
Another option, if you are strong enough to deal with a mass of Kops but do not want to face an angry shopkeeper, is to push a boulder onto the doorway of the shop before you steal.
NetHack's Kops got their name from the Keystone Cops, an incompetent bunch of policemen featured in a series of films from 1912 to 1917.
In UnNetHack, when a Kop is killed, there is a 1/5 chance that another is summoned near the stairs and a 1/5 chance that one appears on a random part of the level. In addition, the Kops can no longer be genocided.
In Slash'EM Extended, shoplifting will generate higher-level Kops with a higher chance, and the player character's level is no longer taken into account. In addition to swarms of Kops, the shopkeeper will also summon soldiers and hostile watchmen from thin air. On rare occasions, an arch-lich is summoned as well, making shoplifting much more dangerous for low-level characters.
Additionally, like in UnNetHack, killing a Kop will sometimes make it reappear on the current dungeon level, or sometimes it will cause the spawn of any random K-class monster instead. Kops do not disappear after pacifying the shopkeeper. Also, killing a shopkeeper or vault guard will surround the player character with a bunch of random K, regardless of who actually killed the NPC.
The Kops are a brilliant concept. To take a gaggle of inept
policemen and display them over and over again in a series of
riotously funny physical punishments plays equally well to the
peanut gallery and the expensive box seats. People hate cops.
Even people who have never had anything to do with cops hate
them. Of course, we count on them to keep order and to protect
us when we need protecting, and we love them on television shows
in which they have nerves of steel and hearts of gold, but in
the abstract, as a nation, collectively we hate them. They are
too much like high school principals. We're very happy to see
their pants fall down, and they look good to us with pie on
their faces. The Keystone Kops turn up--and they get punished
for it, as they crash into each other, fall down, and suffer
indignity after indignity. Here is pure movie satisfaction.
The Kops are very skillfully presented. The comic originality
and timing in one of their chase scenes requires imagination
to think up, talent to execute, understanding of the medium,
and, of course, raw courage to perform. The Kops are madmen
presented as incompetents, and they're madmen rushing around
in modern machines. What's more, the machines they were operating
in their routines were newly invented and not yet experienced
by the average moviegoer. (In the early days of automobiles,
it was reported that there were only two cars registered in all
of Kansas City, and they ran into each other. There is both
poetry and philosophy in this fact, but most of all, there is
humor. Sennett got the humor.)