To be honest with you, I don't really know.
This swampwerewolf thing I didn't even knew it wan an event. I didn't used to visit Mysty Abiss that much and one day I found it there.
I have no clue if I did something for it to appear or nor, but I can tell you it probably needs to be after the payback event, since when you'll go talk to Logan or Bernard (depending of who made the payback event with you) they'll tell they sense there is something wrong with the pack.