Noun
Brython (plural Brythons)
A (historical) Briton: a member of that people that spoke Brythonic languages.
Meditating were my thoughts On the vain poetry of the bards of Brython. Making the best of themselves in the chief convention. Enough, the care of the smith's sledge-hammer. I am in want of a stick, straitened in song, The fold of the bards, who knows it not? Taliesin
Both were created in the 19th century to avoid the ambiguity of earlier terms such as "British" and "Cymric". citation "Brythonic" was coined in 1879 by the Celticist John Rhys from the Welsh word Brython. Source: Internet