Into what creature does Eustace Scrubb transform?

Eustace Scrubb transforms into a dragon in The Voyage of the Dawn Treader. He changes after wandering away on an unexplored island and falling asleep on the hoard of a dead dragon.

The transformation reflects Eustace’s selfish, greedy nature at the time. He has been thinking “dragonish” thoughts, and waking as a dragon forces him to confront the unpleasant qualities he has displayed toward the other travelers. His new form is painful, especially because a bracelet he wore as a boy becomes painfully tight around his dragon limb.

As a dragon, Eustace gradually becomes helpful and courageous. He finds food, assists the Dawn Treader, and fights a sea serpent. Aslan eventually restores him by removing his dragon skin and sending him into a bath. Eustace returns to human form changed morally as well as physically, becoming a loyal member of the expedition. The 2010 film adapts this episode with some differences from the novel.