Dread Horn Tainted Mushroom
Although receiving a mushroom as loot is common, one of the four mushrooms is very rare. I have personally done approximately 200 Dreadhorn Quests solo. I have received approximately 100 mushrooms as loot. I have received about 33 each of three of the types, but only one of the fourth type. Pictures of the mushrooms are below - the rare variety is on the right.
To put this in 'rarity' terms, out of the approximate 200 Dreadhorn Quests I have done, I have received:
To also add to its rarity, many people do not realize it is rare, and simply do not even loot the mushrooms but let it decay. No one lets the Crimson Cincture decay. This mushroom is in very, very slight circulation.