First Filter.
I don't know of anything, but - don't the Harry Potter werewolves turn three nights? Someone from that world remind me if I'm off-base here? Although I thought it was one day before and one day after.
Vampires came through and managed to skip out into the population. We've had both Twilight ones and the ones from home even before we had the difficulty a while back. So... it's not impossible, possibly not even improbable - I don't suppose there's any tracking them?