Lycanthropy is an intrinsic conveyed by being bitten by a werecreature - a werejackal, wererat, or werewolf. It is also conveyed by eating the corpse of one of the above (if you are human, this is considered cannibalism). The message "You feel feverish" is displayed upon acquiring the intrinsic.
The intrinsic transforms you into the animal form at regular intervals. Quaffing a potion of unholy water will do the same. While in were form, you can summon jackals, rats, or wolves as pets. Polymorphing will toggle between forms.
Lycanthropy is generally considered intrusive as animal forms tend to get encumbered by inventory that was no problem before. Werewolves, in addition, will destroy their shirt, cloak, and body armor when transforming to animal form. To remove lycanthrophy, you can pray to your god (it counts as a major problem), eat a sprig of wolfsbane, or quaff a potion of holy water. If none of these options are available, an amulet of unchanging or ring of polymorph control will keep the intrusive changes under control.
Summoning fellow monstersEdit
While in a changed form, you can use #monster command to call for help - summoning a few monsters of the same species as yours. The fellow monsters summoned by using the #monster command will remain tame after you have returned to human form, or if you become cured, effectively becoming your pets.
A very large number of pet jackals, rats, or wolves can be acquired by summoning help over the course of several transformations to the were form. These pets will be very weak, but you can polymorph them into an army of unstoppable monsters (or use them as temporary meatshields).
One thing to note is that calling for help with #monster requires some power - the same kind that the character uses for spellcasting - and if you don't have enough power, you get a "You lack the energy to send forth a call for help!" message and cannot summon any monsters. This can pose a bit of a problem for very low level characters.
Lycanthropy can be beneficial as it confers drain resistance while in animal form, and you have the option of summoning in many tame pets. If you wear an amulet of unchanging, or have polymorph control (from wearing or eating a ring of polymorph control or in SLASH'EM choosing the doppelganger race and reaching level 9) you may choose not to transform into your were form (providing you are conscious and in good health), leaving no adverse effects. Lycanthropy can also be used as a means of overcoming the drawbacks of Vampire feeding when starting the game as a Vampire in Unnethack. By changing into lycanthrope animal form, you can eat food normally for much greater nutrition than you would get by draining blood. You can also eat corpses that are more than 3 or 4 turns old. If combined with polymorph control, this can be an exceptionally useful tactic. Contracting lycanthropy from a wererat also makes your animal form small enough to walk past boulders and pass through iron bars, making this a very useful shortcut in Sokoban.
Lycanthrope is a starting race in SLASH'EM in which you start with Lycanthropy, though you are automatically a werewolf, and cannot choose to be a were-anything else if that is your starting race. However, unlike characters that have been bitten by a werewolf, natural Lycanthropes will remove their armour before transforming, rather than destroy it (unless it is cursed).