Speaking of myths. What about just standard legends or wives tales or superstitions and such? Like never cross an angry Tems path or other things? Do they have any?
So I can't promise any order or coherency, I'm giving it my best shot. Here we go.
There are an abundance of Tems. Just so many of them. More than come from the AUs that those in the loop are aware of. They all behave relatively the same regardless. Some a little more aggressive, but mostly the same overall. But. Every now and then, they all just, disappear. There's still evidence they were there. Things they own, their homes, etc, all stays the same, just no Tems. Except for one. One, lone Temmie. It doesn't speak to anyone, or really do anything other than be. Sitting at the entrance of Tem Street, a dead end road, lined with nothing but the houses of Temmies. Watching. Waiting. It's highly unsettling and no one has figured out why it happens or what it means. But if Tem Street goes quiet, everyone avoids it, and do there best not to make eye contact with the lonely Temmie. The whole city breathes a sigh of relief when the Tems return.
Mirrors shouldn't have solid frames, or be completely frameless. The edges must be “incomplete”. If a mirror is cracked or broken it must be completely shattered into as small of pieces as possible, or melted down. The home or building must then be kept as well lit as possible for three days, to chase out any “darkness” that may have seeped in. This is because mirrors are seen as possible gateways into the void. Letting things out of the void is collectively seen as a very bad idea. A complete frame is considered a full doorway and an invitation. Frameless is to place no barrier between yourself and them at all. Broken/cracked means they can slip in without invitation.
It's widely accepted that most human creature/cryptid mythos probably originated with Monsters in some fashion. A few would like to go cryptid hunting on the off chance that maybe some managed to not be locked in the mountain? Perhaps there is still distant family hiding out there somewhere.
Another monster answer to human stories, large groups of people vanishing suddenly without a trace is usually blamed on the void. It slips through to the material world sometimes, and it is always hungry.
It's generally accepted, among humans that bad things occur in threes. For monsters however, 3 is generally a number with positive associations, 9 even more so. These numbers rarely if ever have negative connotations.
Eat something spicy to stimulate magic regeneration. Avoid sugary things, as they tend to give a quick short boost in regen, but then slow it afterwards.
Placing a candle in the right corner of the eastern most window is good luck for expecting parents. Burning it even more so. If you can replace the candle with a new one, and keep it burning for the full duration of the child soul's development, it is said the child will be especially blessed.
Well, this is all I have for now. Hit me up anytime!