C.S. Lewis's talking mouse in The Chronicles of Narnia is Reepicheep.
Reepicheep is a courageous, sword-carrying mouse who appears most prominently in The Voyage of the Dawn Treader. He serves as one of Prince Caspian's companions aboard the ship Dawn Treader and is known for his chivalric manners, fierce confidence, and devotion to Narnia.
The character is smaller than the other major travelers, but Lewis uses that contrast to emphasize courage rather than physical strength. Reepicheep's greatest desire is to reach Aslan's country, the mysterious eastern region beyond the world's known seas. His quest gives the voyage a spiritual and legendary dimension.
Readers sometimes confuse Reepicheep with other Narnian talking animals, including the badger Trufflehunter and the owl Glimfeather. Reepicheep is specifically the mouse, and his name is associated with one of the series' most memorable heroic characters.