SLASH'EM introduces four new lycanthropes, the werepanther, weretiger, weresnake and werespider. Only the first two are randomly generated, while the werespider can sometimes be found in the Spider caves. Oddly enough, unlike other lycanthropes, the animal form of the werespider is genocidable.
Enemy lycanthropes also function differently in SLASH'EM. All polymorphed monsters, including lycanthropes, revert to their human forms when killed. Unfortunately, lycanthropes will then often quickly turn back into animals, which must (again) be killed to turn them back into human form, in which they can be banished permanently.
Werewolves in SLASH'EM do not always start hostile. Chaotic players may see peaceful randomly generated werewolves.
The werespider is an interesting type; the spider form is considered oviparous, so it is possible to acquire lycanthropy as a werespider and lay eggs that will hatch into tame werespiders.
One must be careful around even wererats in SLASH'EM. Although the early lycanthropes are not strong, the reversion property can make it so you have to kill them many times before they are gone. Because of this, they are about the most dangerous of SLASH'EM's early monsters.
How to kill them
To fight lycanthropes in SLASH'EM, it is most important to be able to do damage quickly enough to destroy the human form before it reverts back (again) to animal form. In addition, there are some other techniques that are useful for fighting them.
Silver is one of the better ways of dealing with lycanthropes in SLASH'EM, as the d20 extra damage can wipe out a lower-level lycanthrope in one hit, and both human and animal forms of lycanthropes are affected by silver. Luckily, there are more types of silver weapons in SLASH'EM, and thus it is more likely to find one. Also keep in mind that silver non-weapon items or ammunition can be equally potent, provided you can hit with them. Note that silver rings will do silver damage when worn (not wielded) as long as you're not wearing gloves.
Ring of protection from shape changers
This ring is considerably more useful in SLASH'EM, where it forces all shapeshifters into their "natural" form, where you only have to kill them once. This includes lycanthropes. As additonall benefits of the ring, lycanthropes are unable to confer lycanthropy in their human form, and they can't summon help. Unfortunately they are as difficult as any ring to identify.
Fighting lycanthropes at a distance is quite valuable, as not only are they unable to confer lycanthropy except through melee attacks, they also cannot summon help at a distance. Also, with multishot bonuses, some roles might be able to do more damage fighting with a bow or daggers than a melee weapon anyway. Multishot bonuses are quite valuable, as again, it matters how quickly you can do damage to lycanthropes. silver arrows are a particularly potent option.
Although their human forms will not respect Elbereth, animal lycanthropes and their summoned friends will, so even an Elbereth written in the dust can give you breathing room and allow you to plan either your escape or the lycanthrope's demise.
The new lycanthropes, specifically
Like most introduced SLASH'EM monsters, the new lycanthropes are far more dangerous than their vanilla counterparts. The werepanther is probably the greatest new threat, as it tends to appear somewhat early (often as early as the mall or Minetown, and often at the The Oracle), it hits very hard, it summons hard-hitting panthers and jaguars, and at speed 15 it can outrun a player without speed boots. For this reason, you should always use the farlook command to identify any feline at a distance, and not be afraid at all to run away. Luckily, no lycanthrope can follow you to other levels, so retreat is usually an option if you are close enough to the stairs.