Cats are a group of monsters commonly depicted in the game with the non-capitalized letter "f", which are notable because proper treatment of felines can be rewarded with an artifact of major power, the Ring of the Master Cat.
To get it, of the following mundane cat-type monsters
the player must not kill any, even one, until he meets the fourth and last possible "f" creature, the cat lord, who is located on D:35 or D:36. If the player succeeds, the cat lord will commend him for respecting the cat race and grant him the ring, then disappear. If the player, however, has killed even one cat, the ring will not be granted. If the player has only killed one cat, the cat lord might be created non-hostile, but if he has killed cats in the same manner as other monsters, the cat lord will attack on sight. For details on this highly dangerous foe, check his article.
The Ring of the Master Cat is powerful enough that many players find it worthwhile to at least try to avoid killing cats unless it is absolutely necessary. Once even one cat has been killed, either by necessity or by accident, there is still a certain incentive to avoid killing more since each killed cat makes the cat lord stronger in combat. On the other hand, this will also increase the amount of experience the player is awarded for killing him. In any case, to avoid killing cats, there are many strategies imaginable, the most obvious being teleport effects, killing cats using pre-existing traps, ordering a pet or companion to kill the cat, or using higher speed to run away. The player can also take advantage of other monsters' special powers, such as making sure a hostile cat is in the blast radius when the PC is hit by a vortex, or letting a monster hit them with a long range breath attack if there is a cat behind the PC. Notably, sacrificing the cat will count against the player, as will (accidentally or otherwise) killing an adjacent cat with an exploding (door) trap.
Cats encountered in the Arena can be teleported out, which counts as a combat win. Throwing potions of exchange at them will change them to a different creature. Cats killed by floor traps do not count against the player, so a wand of trap creation can also be used to remove troublesome cats (be careful, however, as this method requires moving the cat repeatedly onto and off the trap, and it is easy to make a mistake and kill them with a melee attack).