C.S. Lewis called the fictional country in The Horse and His Boy Calormen.
Calormen is a large southern empire in the world of Narnia. The Horse and His Boy, published in 1954, follows Shasta and the talking horse Bree as they flee Calormen and become involved in events affecting Archenland and Narnia. The story takes place during the reign of King Peter, Queen Susan, King Edmund, and Queen Lucy.
Calormen is distinct from Archenland, a neighboring kingdom, and from Narnia itself. Its culture, names, clothing, architecture, and political structure draw on elements familiar from historical and literary depictions of the Middle East and North Africa, though it remains a fictional society.
The book is unusual in the series because its principal human characters begin outside Narnia. It is also chronologically set during the period covered by The Lion, the Witch and the Wardrobe, rather than following that book as a simple continuation.