I have seen these guys everywhere when I was soloing this area as a monk. Easy kill all the time, but good drops. Not best but good. I didn't really notice them out in full moon, but Once I solo area again I will update day/time/moon. I doubt the moon has to be full for these guys. Maybe even darksday.
I don't know if this is true, but these things do seem really rare, is it possible that they spawn only during a full moon? At night of course. Well, if anyone knows or even has an idea thx. Gromit