This spoiler about tame monsters in Nethack and Slash'EM was written by Eva Myers, with helpful contributions from J. Ali Harlow. If you have any comments or corrections, please feel free to email me. Other spoilers about pets which may be of interest are:
There are three different types of tame monster in Slash'EM:
The main distinction between these types of tame monster is that pets have multiple uses, but minions and spell beings are only useful for helping you kill monsters. Here are the differences between them in more detail.
Since spell beings are magical manifestations of the spellcaster's mind, you are held responsible for their activities. If your spell being kills a monster, everything is as if you had killed it - you gain the experience, and if the monster was peaceful, you suffer the usual penalties. If your spell being is in its original form, and it explodes at a peaceful monster but does not succeed in killing it, this will anger the monster.
[The following two points about polymorphed spell beings were observed in wizard mode, but may be bugs.]
Spell beings in their natural form are mindless and will therefore attack anything. If you polymorph a spell being, it will continue to attack monsters of vastly higher level than itself, even if it is polymorphed into an intelligent monster. Also, peaceful monsters attacked by a polymorphed spell being will remain peaceful, whether or not the spell being succeeds in injuring them. (If it manages to kill them, then you will still get the blame, but this seems to provide a fairly foolproof method of killing peacefuls if you are willing to take the penalty but unsure whether you'll survive the combat.)
Not all monsters are capable of being tamed. The Wizard of Yendor, Medusa and your quest nemesis are your implacable enemies and cannot even be made peaceful. In Slash'EM, the following monsters are also untameable: migo queen, Cerberus, Aphrodite, Grund the Orc King, the Rat King, Girtab, Shelob, Father Dagon, Mother Hydra, Doctor Frankenstein, Cthulhu.
The following monsters appear to be untameable for technical reasons: they have their own special structures and cannot also have the structure for a pet. They can be made peaceful, and indeed some of them are peaceful by default.
Demons cannot be tamed unless you are polymorphed into a demon.
In addition, monsters may resist being tamed by a scroll of taming, magic harp, or spell of charm monster or command undead. The probability depends on their magic resistance; it is
(Monster's magic resistance) / (100 + alev - dlev)
where alev is your experience level if you used a spell or 9 otherwise, and dlev is the monster's level or 50, whichever is less. Some monsters (e.g. the Riders) have such high magic resistance that they can never be tamed in this way unless they are first level-drained.
There are seven ways to tame existing monsters:
Polymorphing yourself into a female egg-laying monster and laying an egg is a useful technique for acquiring pets. Any egg laid by you is guaranteed to hatch into a pet. You can also sometimes get pets from eggs which you didn't lay, although this is probably less useful. Any dragon egg which hatches while you are carrying it will become a tame baby dragon. If you are male, any other egg which hatches while carried by you has a 50% chance of yielding a pet. (From the source code, ``The identity of one's father is learned, not innate''.)
If you have lycanthropy, or are playing a member of the Lycanthrope race in Slash'EM, you can ``summon help'' with the #monster command.
Gremlins can multiply in fountains or by falling into water, brown molds and blue jellies may multiply from an attacker's heat, and puddings may divide when hit. If any of this happens to you while polymorphed, or to an existing pet, the resulting monsters will be your pets.
If you are polymorphed into a demon other than a succubus, incubus or balrog, and are fighting bare-handed, you may get some ``hell-p'' in the form of a tame demon.
The Slash'EM Healer technique of revivification can be used to resurrect any monster, and if its chance of resisting the charm monster spell was less than 100%, it will revive tame if possible and otherwise peaceful if possible. The Necromancer technique of raise zombies will raise humanoid corpses as zombies and then attempt to tame them, but they will have the same chance of resisting as if you had cast charm monster at them.
There are other ways of resurrecting monsters, and monsters can rise from the dead on their own (<i>e.g.</i> if they wear amulets of life saving). However, only revivification and raise zombies will tame an untame monster. A pet which rises from the dead has a chance of coming back tame, provided that it wasn't abused while alive.
You can apply a figurine to create a monster. If the figurine is blessed, there is an 80% chance that the monster will be tame, but if it is uncursed or cursed the chance is only 10%. Some people wish for a blessed figurine of an Archon so as to have a powerful pet.
You can also acquire a tame djinni by rubbing a magic lamp or quaffing a smoky potion, but this isn't very useful, especially compared to getting a wish from a djinni.
The spell of create familiar will create a pet from thin air. This pet has a 1/3 chance of being your preferred pet type (cat or dog) and a 2/3 chance of being a random monster. Unfortunately, create familiar is a level 6 spell, and therefore not of much use by the time you are able to cast it.
In Slash'EM, some artifacts can be invoked to summon pets. The Hand of Vecna will summon undead creatures, the Candle of Eternal Flame will summon a fire elemental, and the Storm Whistle will summon a water elemental.