Shoggoths and Giant shoggoths are two of the most powerful monsters in SLASH'EM, although not necessarily the most dangerous. Both forms have very damaging attacks, move quickly, and have an acidic engulfing attack.
Giant Shoggoth Edit
Giant shoggoths are more powerful versions of regular shoggoths. They have a speed of 20 (making them faster than a player in speed boots), an engulfing attack which, in addition to doing a ton of acid damage, corrodes most of the corrodable items in your inventory (which means you will be walking around with corroded wands and keys for a while), in addition to two very powerful bite attacks. They also have a very high base level, meaning lots and lots of HP.
Luckily for those who encounter them, shoggoths are not whirly like air elementals, and thus a wand of digging will work to expel you and set them to one hit point. However, an engulfed player still has to deal with their 8d10 engulfing damage at least once, not to mention their bite and corrosion attacks. If you have few hitpoints, it is best to avoid being engulfed altogether. Note that acid resistance, as granted by a lab coat (similar to an alchemy smock but provides MC 3) or yellow dragon scale mail will protect you against the damage from the engulfing attack, but not from the biting attacks.
Also, shoggoths, unlike other super-monsters in SLASH'EM, are not immune to death rays. Given the greater number of charges on wands in SLASH'EM, an awake and non-hiding giant shoggoth (see below) is well worth a charge or two from a wand of death, or a casting of finger of death. A wand of death is also a good option if you find yourself engulfed and lack a wand of digging. It is also a good idea if you are low on hit points, as the shoggoth may re-engulf you before you can respond if you zap it from inside with a wand of digging.
The shoggoths and their giant variant do not actually pose a threat to a careful player when they normally appear, because, like trappers and rock piercers, they hide until you enter their square. Thus if you have warning or extrinsic telepathy, you will never be taken by surprise by a "natural" shoggoth. And you will, by the time shoggoths can appear. This is probably a very good thing because Demogorgon's lair is full of shoggoths, which would almost be as bad as Demogorgon himself if they weren't avoidable in this fashion.
Unfortunately for some unlucky players, shoggoths are considered a valid polymorphable form. This means that a chameleon can turn into one on dungeon level 9, or any monster could theoretically step on a polymorph trap and turn into one. Since shoggoths are faster than the player, this is very likely to become YAAD.
The fact that shoggoths are considered a valid polymorphable form is very useful for any player with polymorph control and a source of polymorph. As a giant shoggoth, you will wipe out shopkeepers in the early game, but more importantly, the form can be used to rob the Black Market later on. They have enough hitpoints to weather One-Eyed Sam's attacks, and since they are not @'s (and have no heads anyway), they need not worry about the beheading and canceling abilities of Thiefbane. Being immune to stoning, they do not need to worry about the cockatrices that guard the Black Market, and the other monsters there are quite trivial. Giant shoggoths do not make particularly good traveling polymorph forms though, as they are blind, cannot use weapons and armor, and have no hands.
Another use of giant shoggoths is that their corpses (but not those of the normal variety) are capable of conveying teleport control, but also teleportitis. This is not terribly useful in and of itself, as typically one will encounter them only shortly before getting the Amulet of Yendor, which in SLASH'EM prevents all forms of teleportation. However, a careful player may find it useful to reverse genocide them, provided they use some permanently engraved and well-placed Elbereth squares. They have a higher chance than tengu to grant the intrinsic, and always leave corpses. In addition, their corpses are also capable of granting poison resistance, shock resistance, and cold resistance. They do have a high nutritional value, so a player would be well-advised to use a tinning kit or amulet of magical breathing in doing this.
A shoggoth would appear to make a good pet, as they have extremely damaging attacks and move quickly. Shoggoths are also relatively easy to tame, as a regular variant only has a MR of 25 and will grow up into a giant shoggoth, and since they tend to hide it is very easy to walk up to one and repeatedly cast charm monster until they become tame. While a shoggoth is certainly capable of doing large amounts of damage, the fact that they hide, eat both metalic items and regular food, and always have a chance of turning traitor mean that they are difficult to control, especially without a magic whistle. Despite having extremely powerful attacks, even a giant shoggoth only hits monsters as a +0 weapon, meaning that they are completely incapable of damaging many of the more powerful monsters that you will encounter in SLASH'EM. Their only advantage over a Solar is that they are much easier to tame and extremely easy to keep tame, as they will eat almost anything.
(Much of this information was adapted from http://alt.org/nethack/dudley/?314 and posted here for convenience)
Encyclopedia entry Edit
"... in the place of utter blasphemy, the unholy pit where
the black realm begins and the watcher guards the gate -
I saw a shoggoth - it changed shape ..."
It was a terrible, indescribable thing vaster than any subway
train -- a shapeless congeries of protoplasmic bubbles, faintly
self-luminous, and with myriads of temporary eyes forming and
un-forming as pustules of greenish light all over the tunnel-
filling front that bore down upon us, crushing the frantic
penguins and slithering over the glistening floor that it and
its kind had swept so evilly free of all litter. Still came
that eldritch, mocking cry -- "Tekeli-li! Tekeli-li!" and at
last we remembered that the demoniac Shoggoths - given life,
thought, and plastic organ patterns solely by the Old Ones ...