The first kill that the player makes in each game has a special significance to two quests: the Filk Quest and the Courage Quest. While neither of these quests are necessary to complete the game, the Filk Quest is necessary for ultra endings, and the Courage skill is generally fairly desirable for many PCs. Unfortunately, the first kill must be decided long before the PC undertakes either of these quests, so judicious choice of first kill can help minimize future grief. Note that the gender of the first kill does not seem to matter (e.g. a female kobold just counts as a kobold).
The Filk Quest Edit
The Filk Quest requires the PC to descend to a depth of the Infinite Dungeon equal to that of the number of times the player has killed the first kill monster. Generally, long descents (particularly >60 levels) in the ID can be extremely dangerous and corrupting, and so players wishing to undertake this quest but avoid the hazards of a deep descent into the ID are generally advised to take a rare monster for their first kill.
The Courage Quest Edit
The Courage Quest requires the PC to kill 20 of the monster of their first kill after talking to the Old Barbarian. This is most easily achieved by choosing a very common monster as the first kill, so as to minimize the difficulty in completing this quest.
Suggestions for the First Kill Edit
There are essentially three schools of thought as far as choice of first kill is concerned: choose an extremely rare monster, choose an extremely common monster, or choose a relatively rare monster.
Extremely rare monsters are ones that are unique, or nearly so. These include NPCs in Lawenilothehl such as the crime lord, half-orc bouncers, the cloaked ratling or the mad doctor--indeed, many monsters in Lawenilothehl are excellent choices: beggars are also only found in this location as well, and thus minimizing the number of kills of these is fairly easy. They also have the advantage of being considerably easier to kill than some of these other NPCs. Townsfolk in Terinyo, a raider lord, a cute dog, or (ironically), an Old Barbarian are also choices that allow for easy completion of the Filk Quest.
Players who have no interest in completing an ultra ending may simply wish to kill an extremely common monster such as a goblin, orc, or rat. This will allow them to complete the Courage Quest with ease, since players can reasonably expect to encounter many of these monsters over the course of a game. Players may wish to begin the game by visiting the Small Cave, since there are normally a number of common monsters in the starting room.
Finally, players who wish to have the option of completing either quest try to pick a monster whose numbers can be easily controlled. This is ideally a monster that is rare in most areas of the game, but is relatively common in certain specific places. Examples of these sorts of monsters include beggars and muggers, who are only found in Lawenilothehl; giant frogs, who are generally uncommon but can be found in large numbers in swamps; assassins, outlaw leaders, bandits or outlaws who are found in Lawenilothehl, uncommonly in regular play, but can be generated in the Assassins' Guild or in Wilderness encounters, respectively; chaos rats, which are generally uncommon but can easily be created if there is access to a corruption trap. Any other monster that is somewhat uncommon, such as a huge bat, may also be used, although since these monsters cannot be generated easily in abundance, it will delay completion of the Courage Quest. In the ideal case, this strategy would result in exactly 21 kills of their first kill monster, allowing for a fairly painless ID dive to complete the Filk Quest, and 20 kills for the Courage Quest. Seeking out a giant frog in the swamp would probably be the best bet for a player aiming for an Ultra ending while still doing the Courage Quest. A rabid dog is a bad choice for anyone who wants to do the Courage Quest because of rabid dogs' rareness in the later game (ID-hopping for them could take a really long time).
Remembering the First Kill Edit
Players concerned with their first kill are advised to make note of it in some fashion. The easiest way is to enter the name of the monster in the monster memory by pressing the '&' key. The player can then check their first kill at a later point by pressing '&' again. Some players also rename their PC after their first kill. Writing down the monster name is also another obvious option.
If the player has forgotten the first kill, it is possible to research it using the Courage Quest.