It is unknown when Buellt began to emerge as a distinct political unit. Its organization as a cantref seems to have developed along the earlier tribal boundaries of a
gwlad ("people") or
tud ("tribe"). During the
Early Middle Ages, Buellt and
Gwrtheyrnion on the other side of the Wye formed a small regional kingdom. This kingdom's rulers traced their descent back to the legendary 5th-century warlord
Vortigern (, from which Gwrtheyrnion was named.) The
Historia recounts that after Vortigern had invited the
Anglo-Saxons to Britain and then been forced west, his son
Pascent ruled Buellt and Gwrtheyrnion as a grant from
Ambrosius Aurelianus. Whatever the reality of this story, another section attributes descent from Pascent and Vortigern to
Ffernfael ap Tewdwr, a 9th-century ruler of Buellt and Gwrtheyrnion known from other sources. The
Historia further includes Buellt in its
mirabilia section, or list of marvels. According to the text, Buellt is the location of the "Carn Cabal", a (since lost)
petrosomatoglyph: the imprint of a dog's paw. This marvel is attributed to
King Arthur's dog
Cavall, who supposedly left the print while chasing the boar
Troyt. Afterward, Arthur placed this stone on top of a
cairn, and anyone who tried to move it would find it back in its place the next day. The placename survives as
Carn Gafallt near
Rhayader, though it is unlikely the modern hill is the same one mentioned in the
Historia. The prose tale
Culhwch and Olwen contains a more elaborate version of Arthur's hunting of the divine boar, here known as
Twrch Trwyth, however, in
Culhwch the boar's detailed itinerary does not take him through Buellt. ==Norman era==