Ceidio, in the promontory of Lleyn, is under the remarkable isolated hill of Carn Madryn, which takes its name from Madrun. The local tradition is that on the burning of the palace of Gwrtheyrn, under Tre’r Ceiri, Madrun fled with Ceidio, then a child in arms, to the fortress on Carn Madryn; and that later in life Ceido founded the church that bears his name beneath the mountain.
[...]
About half a mile east of Rhayader, in Radnorshire, there is a barrow, in a field called Cefn Ceidio, under which it is supposed that he has been buried.
Lives of the British Saints, v2, by Sabine Baring-Gould and John Fisher (1908).
And just as an afterthought, Ceido was associated with sanctuary seeking deer, as his horse was one of the 'cloven-hoofed horses' of Britain.
ref; Breverton's Book of Welsh Saints...
Wow cloven hoofed horses, I'd not heard of those. That's an excellent train of thought, thank you.