Истрад-Тиви
In the first branch of the Four Branches of the Mabinogi, the tale "Pwyll Pendefig Dyfed" is told of the son of Pwyll, Pryderi, following his father on the throne of Dyfed and adding the three cantrefi of Ystrad Tywi and the four cantrefi of Ceredigion to his kingdom.