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) they may drop specific extra loot.
At the time of writing this entails:

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.
Gorlim
    His accursed helmet.
The Stormbringer
    Drops itself as (random-artifact-like) ego weapon, ie usable item.
Eol
    The Claymore 'Anguirel'.
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 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.
Zu-Aon
    A potion of learning and a scroll of artifact creation.


Further, even without any flags and independant 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.