Sighted between (H/I-8) during Full Moon (95%) on Iceday between 0:00-4:00. Either it's triggered by the Lunar Phase, day of the week, or it has a high repop timer. It wasn't up the following 2 nights, and since no one killed it when it first spawned, I'd imagine if it goes unclaimed past 4:00, its respawn timer is reset.
Such a shame no one was around to kill it with me.