This is my take on what a brythonic polytheist is and others may well disagree.
First a polytheist is someone who believes in the existence of many gods. In my experience they usually follow a small number of these gods and will acknowledge the existence of many others.
Brythonic – the free dictionary gives a definition of brythonic as an adjective of “Of or relating to the Brythons or their language or culture.” and as a noun of “the S group of Celtic languages, consisting of Welsh, Cornish, and Breton “. I think most dictionaries would give similar definitions.
It is the former defintion which applies the most to my interpretation of what a brythonic polytheist is. Historically the Brythons were the peoples that inhabited the mainland of Britian before the Romans came. It is known that the majority of these peoples spoke a range of p-celtic languages. The modern descendants of these languages are Welsh, Cornish and Breton. According to linguists Pictish is generally agreed to have been a p-celtic language.
Using the defintions of brythonic and polytheist given above I would currently describe a brythonic polytheist to be one who believes in the the gods of the peoples living in mainland Britian before the Romans invaded. In my case I am particularly interested in the gods that may have been known and worshipped in what is now the South West of Scotland.
The tough part of that is finding out about those gods and developing relationships with them!