Was curious, checked the monster with Monster Modder (comes with Gold Box Companion), and behold:
...
Giant slugs have 4 plate mails and 2 composite long bows, giant snakes have 8 leather armors, giant spiders have leather armors and a shield...
Interesting, and yes you are right, the loot from Shambling Mounds included a magical longbow (instead of a crossbow), and of course the shield + 1... how could I forget this source of magical shields which helped to equip all of my party member with one of these things
... but does that mean that this loot is intended I wonder... I don´t use GBC myself, and like I said, not even the official cluebook does mention these items. Also they are way too overpowered compared with the other magical gear you find in the game.
Also, some things I forgot to mention regarding the game in general (can´t seem to edit my previous post):
-Enemy encounters were more varied than I remembered. The battles remained relatively interesting throughout the game. Also some enemies you encounter mostly (or exclusively) in certain places in the wilderness, which highlights the danger some terrains pose
-Regarding the huge amount of time that passes when you wander about the worldmap, I did not care and wandered throughout the map, and none of my chars even aged one year (so the characters aging in accordance with the time spent in game world is either not implemented, or I didn´t spend enough time wandering around..). And I didn´t use the Haste spell - not only was it not really needed, but I also don´t like the age counter of my characters go up
-The main villain I thought was well designed. Him playing with the party (instead of him destroying all of them in the beginning) made kind of sense if you take into account how strong he actually is. Also he departed with style
-The end in general felt unorthodox, but in a refreshing way
So definitely recommended, but the successor has a somewhat dull look compared to this game, or maybe it needs some acclimatization to the newer interface I don´t know, will try it in the next weeks I hope