Blue Orange
Gone to Texas
Well, if we're talking iconic villains, then the druids also have to be in the DMG, since they're decidedly evil.
Facts to support this statement:
1. They have classically been depicted as human-sacrificing, especially with the wickerman which was used to burn sacrifices alive in great agony.
2. They're British. All villains have British accents. QED.
Of course, all we know about them is from Julius Caesar, who was trying to wipe them out. (This is a big problem with a lot of ancient history...only a few sources survive and they are, of course, extremely partisan.)