Dryad

Dryads (also can be refered as "Wood Nymphs", "Wood Gods", "Wood Goddesses", "Hamadryads", "Tree People", and "Silvans") are spirits of trees. They are able to assume physical bodies identical 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 then dissappear into thin air.

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

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

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

Prince Caspian
When Narnia is invaded by the Telmarines, they began to cut down trees, resulting in the dryads falling asleep within their trees for centuries. Lucy Pevensie sees them waking and dancing sleepily shifting back and forth from treeish to human-like form and this happens because Aslan was nearby in the clearing. Some time later they are fully awakened by Aslan and dance about Aslan, Lucy, and Susan Pevensie pay adoration to Aslan by crying out his name. The dancing is joined in by Bacchus, Silenus and his donkey, and the Maenads. The next day they head off to Aslan's How and during the battle they swoop in on the Telmarines using the braches and roots to swing them around, strangle them, and destroy their weapons. During the celebration of the Old Narnians victory, soils are dugged up for them to eat. some Holly dryads are mentioned drinking wine and they become very chatty.

The Voyage of the Dawn Treader
Reepicheep tells of a dryad telling him the verse for the Utter East while he is still in his cradle.

The Silver Chair
A dryad is mentioned looking after Jill during her stay at Cair Paravel. Later on, dryads are seen performing the snow dance with fauns, dwarfs and other woodland animals.

The Last Battle
A beech dryad leaves her tree and warns King Tirian of how the trees in Lantern Waste are being felled and the death of their dryads as a result. She soon falls down, dies, and vanishes into thin air, showing that her tree has 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 (execpt 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. Also, in a deleted in scene from the second film depicts a dryad 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, his leaves exploded rather than falling on the ground dead and disappearing into thin air.