A wide chasm. Riches rumored to be on the opposite side. Murders of crows cawing and hopping about on this side. If you sleep near the crows they form a bridge in your dreams that you can walk across the ravine. If you wake while walking the crows fly away leaving you to fall.
The bridge of crows is from an actual fairy tale I haven't read. I just saw an illustration of it as I hunted for silhouette material. It showed up in my dreams chart. I suppose it's more a location or feature than a monster.
I think I would make it so that one or more players would have to find a way to fall asleep near the crows, even with all the noise. And it would take 1d6 turns for them to see the bridge and cross it. Hopefully there is some encounter in the mean time to make things tense. Maybe something lairs right next to the crow landing.
Crossing the bridge successfully would essentially teleport the character to the other side.
So, I think that's about the end of my fairy tale run. I was trying to think of some more direct threats, monsters pcs could actually fight. But from my experience fairy tale threats are either beaten by simple rules or gobble you up whole-- not much exciting combat going on. If you've got ideas post them in the comments.