Ystrad Tywi
Seisyll ap Clydog was King of Ceredigion in Wales some time in the late 7th or early 8th century. He gave his name to the later kingdom of Seisyllwg, which consisted of Ceredigion plus the region known as Ystrad Tywi; as such he was possibly the king responsible for the expansion. However, there is no contemporary evidence of this, and almost nothing is known of his life or reign.