A gnoll is a type of monster introduced in SLASH'EM. They have relatively damaging attacks and are generated with weapons. Gnolls are always generated hidden, so if you are able to detect them through telepathy it may be wise to avoid them completely. Higher level gnolls can be generated with dragon scale mail, but all gnolls are too large to wear body armor. None are considered a valid polymorphable form.
Gnolls also appear in other variants, and are quite different in each variant.
Ordinary gnolls are generated with orcish equipment. High level characters have a high chance of running into them in the Mine King's level, where they will be much more difficult opponents than the regular gnomes. One should always either use telepathy to detect them from afar, or avoid apparently blank spaces in the Mine King's room.
Gnolls can grow up into gnoll warriors through level gain. Randomly generated gnoll warriors have a 5% chance of being generated with orange dragon scale mail, 1.667% chance of being generated with splint mail, and otherwise scale mail. They also have a 66% chance of being generated with a katana.
Gnoll warriors can grow up into gnoll chieftains through level gain. Randomly generated gnoll chieftains are guaranteed an orcish helm, orcish shield, a katana, and a random offensive item. They have a 10% chance of being given blue dragon scale mail and a 90% chance of crystal plate mail.
Gnoll shamans have a 10% chance of starting with silver dragon scale mail, 72% chance of starting with crystal plate mail, and 18% chance of starting with red dragon scale mail. They are guaranteed an athame, two offensive items, and 12-15 shuriken.
Gnoll shamans are the only randomly-generated monsters in any version to use both arcane and clerical monster spells.
In dNethack, gnolls are almost entirely different from their counterparts SLASH'EM. They are not represented by G, and have different stats. Gnolls can still be found in Yeenoghu's lair in dNethack.
In GnollHack, gnolls are a playable race. They also exist as monsters which are very different than SLASH'EM's gnolls.
The gnoll is a Dungeons & Dragons creature resembling a humanoid hyena. Many serve and worship the demon god Yeenoghu.
Gnolls were an early creation of D&D, and are described in the first manual, released in 1974. Irish fantasy author Lord Dunsany (1878-1957) was given credit for inventing the word "gnoll", or, as he spelled it, "gnole", in a short story published in 1912 called "How Nuth Would Have Practised His Art upon the Gnoles".
D&D gnolls are somewhat different from Dunsany's "gnoles". Dunsany does not give a description of what his "gnoles" look like, but portrays them as secretive but dangerous creatures that guard a hoard of emeralds in a house in a forbidding forest. The gnolls of D&D, by comparison, are described as aggressive desert-dwelling nomads that actively raid and plunder other settlements.
Early D&D manuals state that gnolls are a cross between gnomes and trolls, which implies that the name "gnoll" is a portmanteau of the two words. However, the authors acknowledge that Dunsany never stated what the inspiration was for his word "gnole", and later manuals do not describe gnolls' pedigree. SLASH'EM includes them in the gnome monster class, but does not consider them to be gnomish for purposes of cannibalism, so gnome players can eat their corpses without an alignment penalty. (Their corpses are poisonous, however, so one should avoid eating them without poison resistance.)