drop rates?

[Mattius]

Is there anything on the wiki regarding the likeliness of uncommon or rare drops or things of the sort? I've tried their search.. I noticed sometimes you can visually see a mob wearing an item and after you kill him you find to note that he had a unique item, like a greater scalebone with a unique wand for example. I've killed hundreds to possibly thousands without getting something unique. How often does it happen from mobs? or bosses?