Potion, amulet, scroll and ring drops
AFAIK, dying monsters can only drop items they had in their inventory. As a result, no monster ever drops a scroll, amulet, potion or a ring (with the exception of the king of the orcs and his bodyguards). They can occasionaly be found on the floor, but it still makes those items excessively rare. Maybe some monster types should carry those types of items?