Nevertheless, there are a couple of intriguing - and perhaps interlinked - clues:
Peibo Clafrog (aka Peibio Clafrog, Pepiau Glavorawc, Peibiawn Glawrawc, Pepianus Spumosus) ruled Ergyng a generation before St Tydecho was knocking about. His epithet is generally translated as 'scabby', 'dribbling' or 'leprous', and relates to the fact that he couldn't stop dribbling.
He tried to drown, then burn alive his pregnant daughter Efrddyl. She not only survived both with barely a blink, but gave birth in the pyre, calmly nursing her baby boy as the flames licked harmlessly around them. The baby became St Dyfrig - contemporary bishop with Tydecho - and cured his psychopathic grandfather's affliction. (St David also nipped by to cure Peibio's blindness.)
Garthbeibio and Ynys Beibio are both said to be named after him. Though no records explain why a ruler in what's now Herefordshire, Gloucestershire and Gwent should have scattered his moniker over landmarks in Powys and Gwynedd.
Peibio, as mentioned in the Welsh legend Culhwch and Olwen. He and his brother Nynnio were both insane kings, who weren't content to rule only their kingdoms, but wanted to own the galaxy too. The giant Rhita Gawr - whose stomping ground was Snowdonia - had enough of their arrogance, so turned them both into oxen.
In the 17th century, the bard Iolo Morganwg composed Triads about the legendary Hu Gadarn, who shackled Peibio and Nynnio to yokes, in order to use their strength to drag the monstrous Afanc out of the Lake of Floods. Thus Wales was saved from inundation and the Welsh were introduced to the concept of ploughing.
Though half of Wales claims to have the afanc's lake in their vicinity, there is one contender close by Garthbeibio. Llyn Llion is sometimes said to be an alternative name for Llyn Tegid, or Bala Lake, just over the mountain. From there the creature was said to have been dragged into Cwn Ffynnon, which is located above Llanmawddwy, just up the valley and around the hill from where Gwely Tydecho may be found.
So maybe an obscure Beibio link lies there, lost in the depths of a remote mountain lake. After all, the Afon Twrch flows by there too.