Ok, I was just on my chocobo riding through La Theine Plateau when I saw him run by me! I tried to catch up but he ran up a slope and I just couldn't catch him. Why's this guy still around if he's from a 2005 fest?
These guys' WS is called Aetheral Toxin. It doesn't deal any damage, but gives you the effect of Dia, which can be pretty dangerous at level 1 without any means to cure yourself.
The best part about it though: Instead of the usual "Character receives the effect of Dia." message, you instead get "Character is Dia." which I find to be pretty funny.