TomeNET

(6.3a) Special item drops of monsters                                           
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Some monsters have the DROP_CHOSEN flag. This means that in addition to the
normal loot (see (6.3) for loot tables) and dungeon boss special drop, they
may drop specific extra loot. At the time of writing this entails:

Halloween townies (only during Halloween seasonal event)
    Fortune cookies.

Smeagol
    A ring of stealth.
Robin Hood
    A long bow.
Dragonriders
    Firestones, normal or small.
Raal's Tomes
    (Rarely) a tome of udun or an elementalist's or destroyer's handboook.
The Stormbringer
    Drops itself as (random-artifact-like) ego weapon, ie usable item.
Gorlim
    His accursed helmet.
Eol
    The Claymore 'Anguirel' or the Gauntlets of Eol.
Artsi
    The set of cesti 'Fist of Chaos'.
Kronos
    His sickle of unmatched sharpness.
Gothmog
    His terrible, fiery whip.
Mardra
    Her dragonrider coat.
Great Wyrms
    The amulet of Grom, the Wyrm Hunter...
Vlad Dracula and Mephistopheles
    Drop a random, heavily cursed true artifact if available.
Saruman of Many Colours
    The Star of Elendil and a Mage Staff of Wizardry (+10).

Nazgul
    Their specific (yet randomly generated) Ring of Power each.
Morgoth
    His two-handed hammer Grond and his massive iron crown.

Bahamut
    A rod of healing of the istari.
The Living Lightning
    A randart lightning ring that grants speed, an air tome, a rune of mana and
    a rune of lightning, a rod of lightning balls of the istari and
    a randart sky dragon scale mail.
The Hellraiser
    A potion of learning and a scroll of artifact creation.
Dor
    A rod of havoc of the istari and an energy cell.
Zu-Aon
    A potion of learning and a scroll of artifact creation.

There might also be unexpected drops that are not listed, of very special foes.

Further, even without any flags and independent of their r_info.txt loot table
monsters may tend towards item drops that fit their appearance and style.
This means that you will mostly see swordmasters drop swords, mages drop an
occasional mage staff, runemasters drop an occasional rune, monsters that are
martial artists rather drop light armour instead of heavy armour etc.