Spirit

Spirits are beings that are not fixed to a particular form or body.

The following list of the world of Narnia Spirits:

Dryad
Are tree nymphs that are capable of leaving their trees and assuming a physical body almost identical to human form. If a tree is cut down, the Dryads will die and enter to Aslan's Country to enjoy peacefully.

Hamadryad
Wood spirits similar to dryads, but use their own trees to assume a body resmbling a human form, Lewis describes a birch hamadryad as having "a soft, showery voice and would look like a slender girl, with hair blown all about her face, and fond of dancing" and an oak hamadryad as looking like "a wizened, but hearty old man with a frizzled beard and warts on his face and hands, and hair growing out of the warts". A whole swarm of hamadryads dance sleepily around Lucy and Aslan when they are reunited in Prince Caspian.

Horror
Horrors were ghost-like creatures that were entirely black in color, and sometimes could not be seen at night. They were not stealthy, however, for they preferred making loud, terrifying cries, and jumping into battle with their vicious claws. They were also called Poltergeists.

Naiad
Spirits of the rivers, lakes, wells, and streams. They were led by their father and leader known simply known as "The River God", who is described by Lewis as having a "great wet, bearded head, larger than a man's, crowned with rushes.",though the term "river god" is usually titled to male naiads in general. When a stone bridge was built over a river, the naiads in it were imprisoned in the river. They were capable of marrying human men and women and able to sire human children.

Nymph
Spirits that live in the trees and waters, they were capable of leaving their tree or watery domains and assuming a physical body resembling human form. This term is usually used for female Dryads, Hamadryads, and Naiads.

River God
Are water spirits of the same species as the naiads, but are male in form. River gods, as their name goes, only inhabit rivers while water nymphs inhabit rivers.

Tree People
Are wood gods and goddesses who use their trees as their physical bodies. Lewis doesn't specify whether they are the same as dryads or just a different sort of wood spirit, but they are described as beautiful giants and giantesses who have branch-like hair and fingers. They have roots for feet and wade in the earth like humans do in water.