C.S. Lewis’s childhood imaginary country was called Boxen.
Lewis and his older brother Warren created Boxen during their childhood. They combined stories, maps, histories, and illustrated scenes to build an elaborate imaginary world populated largely by anthropomorphized animals. The project gave the brothers a shared creative activity and helped Lewis practice constructing fictional cultures and narratives.
Boxen is not the same as Narnia. Lewis developed the Narnia books decades later, and Narnia has its own geography, history, peoples, and cosmology. Nevertheless, the childhood habit of inventing countries and recording their histories formed an important early background to Lewis’s later fiction.
Lewis later wrote about his childhood imagination in Surprised by Joy. The Boxen material was preserved through the brothers’ papers and was published posthumously, allowing readers to see how his early world-building preceded his mature literary work. The name is sometimes written as “Boxen” rather than treated as a general term for his childhood stories.