Give mobs a "loot table". When a mob comes into the world it would have some
number of items from that table. Most of the time you would get a so-so item,
and maybe one in ten you would get the really good item.
You could also make it such that coveted worn/wielded items don't show up by
looking at the mob, so that people don't wait for the mob to have the item
before trying to kill it.
Then you could make the mobs more easily killable so a wider variety of
players can have the satisfaction of killing it.
-Tankster