Fairy tale monsters

[Naez]

I was talking with a friend the other day about old legends and how monsters always seemed to follow weird rules, like vampires not entering without an invitation, fairies obsessively counting grains of salt, brownies requiring oats and honey, etc. And I'd really love to give my game the Grimm-like feel where figuring out the nonsensical rules is a better solution than fighting.

So far I've stumbled across the vampire (duh) and the hellcat (invisible in the light but glows in the dark) and wanted to find other things with fun abilities/weaknesses like that what else can I throw at my players?

Note: The party in question is level 9 but throw the lower CR ones at me too, might use them in later campaigns.

[BowStreetRunner]

A Redcap (MM3) is a nasty piece of work that believe rivers contain water spirits that can snatch their souls, are suspicious of even numbers so travel in odd numbered groups and only carry odd numbers of coins, and when they die they leave behind a single tooth that grants a bonus when dealing with other Redcaps. CR 2 to 7.

[Palanan]

The kappa from Bestiary 3 is perfect for this. It's dangerous as long as the bowl of water on its skull is full--but if you manage to empty it somehow, the kappa is reduced to whimpering and begging for mercy.

[Afgncaap5]

You could always reskin a regular monster with a rule like that. A standard CR 10 or 11 "boss fight" monster, but given a strange rule that makes it truly wicked before then. "Can't be harmed but when near the warmth of a hearth" or something like that, for instance.