To categorize the creatures of folklore, I've decided to use the system from "Arthur Spiderwicks Field Guide." Despite it being modern fiction, the book was an important part of my childhood, and I still feel that it's categorization system (based on which environments the creatures are said to inhabit) is one of the most useful systems I've ever seen.

In the home

Changeling
Church grim
Church nisse
Cockatrice
Devil

Drawing doll & Spiritus - Devilish little creatures who can gather treasure and fortune for their owners, typically by stealing it from other people.

Elder crone
Elder people
Farm nisse
Fylgje

Hide-runner - A kind of shapeshifter whose spirit can exit the body in the shape of an animal.

Lindworm
Nisse
Plague bramble
Salamander
Shape runner

Wardbeast - A protective spirit in the shape of an animal, which follows a person around, warning them of danger.

Well Man - A figure, dark as a shadow, said to live at the bottom of the well. He can send his animals out to harm people and cause disease, or he can physically grab people who get too close to his well and drown them.

In field and forest

Aeven
Alder people
Alder Stinger
Cliff King
Cloth oak
Elf
Forest people
Green Hunter
Land vættr
Saggy tits
Troll
Worm king

In mountain and hill

Dragon
Dwarf
Elf-shot
Giant
Jotnar
Little people
Mountain people

In lake and stream

Bog crone

Hazel ram - A shapeshifting ram who enjoys playing tricks on people and throwing them in the water.

Nøkke
Stream horse
Stream man

In the sea

Gam
Merfolk
Sea jotnar
Sea troll
Sea monk
Sea worm
Sea serpent
Ship nisse

In the night

Creaking wagon
Ghost
Ghost mancer
Grave sow
Hel horse
Helrak

Long ghosts - A type of ghost that is unnaturally tall and shows up on dark country roads.
Mare
Milk hare
Nightjar
Shapeshifter
Torch man
Valravn
Werewolf
The Wild Hunt
Witch