Dryad

Dryads (also referred to as "Wood Nymphs", "Wood Gods", "Wood Goddesses", "Hamadryads", "Tree People", and "Silvans") were spirits of Narnian trees. They were able to assume physical bodies similar to human form, and the same size as normal humans, and they could go in and out of their trees.

Male dryads lived in Oak, Elm, Holly, and Rowan trees, while female dryads lived in Beech, Silver Birch, Larch, and Willow trees. Each individual dryad was spiritually tied to one tree and if that tree was cut down or destroyed, the dryad tied to it would fall down, die, and disappear.

The Magician's Nephew
Dryads were mentioned coming out of the trees, along with fauns, satyrs, and dwarfs. Aslan summoned the dryad of an oak tree to be a member of the council.

The Lion, the Witch, and the Wardrobe
Dryads were mentioned by Mr. Tumnus as one of the participants in the Bacchanalian festivals in Narnia before the usurption of the White Witch. Tumnus also mentioned that some Dryads were on the Witch's side and posed as spies while in their tree forms. Dryads were seen amongst the members of Aslan 's camp and served as harpists along with Naiads. Both were later seen running away in fear when the wolf Maugrim came into the camp in attempt to kill the Pevensies. Dryads were also amongst the stone prisoners of the White Witch in her castle and were freed by Aslan.

The Horse and his Boy
Tumnus told Shasta (whom he believes to be Prince Corin) of how he hoped to see him in Narnia for the summer dances with the other fauns and dryads.

Prince Caspian
When Narnia was invaded by the Telmarines, they began to cut down trees, resulting in the dryads falling asleep within their trees for centuries. Lucy Pevensie saw them waking and dancing sleepily, shifting back and forth from treeish to human-like form; this happened because Aslan was nearby in the clearing. Some time later they were fully awakened by Aslan and dance about him, Lucy, and Susan Pevensie, paying adoration to Aslan by crying out his name. Bacchus, Silenus and his donkey, and the Maenads joined in the dancing. The next day they set off to Aslan's How, and during the battle that ensued, they swooped in on the Telmarines, using the branches and roots of their trees to swing them around, strangle them, and destroy their weapons. During the celebration of the Old Narnians victory, different types of soils were dug up for them to eat. Some Holly dryads were mentioned drinking wine, and they became very chatty.

The Voyage of the Dawn Treader
Reepicheep told of a dryad singing him the verse for the Utter East while he was still in his cradle.

The Silver Chair
A dryad maiden with willowy-looking hair was mentioned looking after Jill during her stay at Cair Paravel. After Jill, Eustace, Puddleglum and Rilian escaped the Underland, dryads were seen performing the Snow Dance with fauns, dwarfs and other woodland animals.

The Last Battle
A beech dryad left her tree and warned King Tirian of how the trees in Lantern Waste were being felled by Calormene, and the dryads were being slain. She then fell down, died, and vanished into thin air because her tree had been chopped down.

Trivia

 * Evil Dryads were seen in concept art of the Disney version of The Lion, the Witch, and the Wardrobe.
 * Lewis describes the dryads of Narnia as having specific characteristic features of each kind of tree an individual dryad is bound to (beech dryads as queenly, gracious, and goddess-like, birch dryads as slender, graceful, and limber, oak dryads as wizardly and elderly looking, etc), wearing clothes matching the colors of the bark or foliage of their respective trees (beech dryads wearing fresh transparent green, birch dryads wearing silver, and larch dryads wearing green so bright it looked almost yellow), and having leaves growing their heads. He also describes them has being able to move with their trees and cause their trees to take on their human-like appearances.
 * In the films however, dryads are portrayed as non-physical spirits who use leaves and flower petals to form and solidify their bodies, and when they are possessing their trees to move, their trees do not take humanish features (except for one moment when a tree is killed with a boulder, it makes the face of a person crying out in agony within its foliage) when it dies, they also have the ability to move and stretch out their roots like tentacles when fighting the Telmarines.
 * In a deleted scene from the second film, Prince Caspian, a dryad is shown forming and coming out towards the Pevensies and Trumpkin, but dies because his tree miles away was cut down. It is curious to note that when the dryad died, its leaves exploded rather than falling on the ground dead, and slowly fading into thin air.
 * In the Prince Caspian book; a wicked Telmarine man, who was beating a boy with a stick, was turned into a tree by Aslan. Whether this means he was turned into a Dryad, or just a simple ordinary tree, is unknown.