In literature, fog and mist symbolises a variety of related things: obfuscation, mystery, dreams, confusion and a blurring between reality and unreality.
Neil Gaiman uses almost all of the symbolic meanings of fog in the following passage:
They felt the castle long before they saw it, felt it as a wave of sleep that pushed them away. If they walked towards it their heads fogged, their minds frayed, their spirits fell, their thoughts clouded. The moment they turned away they woke up into the world, felt brighter, saner, wiser.
The queen and the dwarfs pushed deeper into the mental fog.
— The Sleeper And The Spindle by Neil Gaiman
Hedgehog in the Fog by Sergey Kozlov, Yuri Norstein & Francheska Yarbusova
HIDE AND SEEK FOG BY ALVIN TRESSELT AND ROGER DUVOISIN
For more on symbolic archetypes in children’s literature, see here.