Werewolves
A key part of the mythos is that werewolves come out when the moon is full. If you invoked screaming terror every time you were seen and had to hide so as not to be hunted, you’d take to coming out to find food and such only at night. During the day, you’d stay out of sight of villages.
Back when the fire was the only way to see through the darkness, you wouldn’t see a whole lot of people leaving their homes after nightfall. If you hear something in the forest, you have to go run-up to it with a torch in hand to see what it is — not many people would leave their house to see what is probably an animal in the middle of the night. That’s just asking to be attacked by wild animals. But on full moons, the landscape is illuminated so people go outside or can see outside through their windows without the need to be standing right next to something — thus, it’s only on full moons that any solitary people who avoided human contact were seen.