A new one-off Big Bad created just for a Filler arc. Not the most dignified place for the villain, because that means the villain will have less effect on the actual canon than the weakest Mooks in the real arcs. This puts the writers in an awkward position because they have to somehow make the villain a credible threat to the heroes, yet at the same time maintain that the threat is relatively new or extremely region contained to explain why such a huge threat wasn't a problem for the rest of the series - rather problematic if the heroes are already facing the upper tiers of the Sorting Algorithm of Evil in canon.