Shasta's twin brother in The Horse and His Boy is Corin. Shasta is later revealed to be Prince Cor, the long-lost identical twin of Prince Corin of Archenland.
Cor was kidnapped as an infant because of a prophecy that he would one day save Archenland from a great danger. After a failed attempt to transport him, the baby drifted ashore in Calormen, where the fisherman Arsheesh raised him as Shasta. He grows up believing he is an ordinary fisherman's son.
During his escape north with the talking horse Bree, Shasta is mistaken for Corin in Tashbaan because the twins look alike. Corin, known as “Corin Thunder-Fist,” is a bold young prince who enjoys fighting and adventure, while Shasta is more cautious and unfamiliar with his royal heritage. Their reunion helps establish Shasta's true identity.
The brothers are easy to confuse with Cor and Corin: Cor is Shasta's true name, while Corin is his twin. After Shasta warns Archenland of the Calormene invasion, he takes his rightful place as heir.