Cornwall, the land of the Cornish Welsh. often appears in Arthurian tales.
In Britain, in the 5th/6th century, the south-west of Britain, including modern Cornwall, Devon, and at least part of Somerset, was known as Dumnonia. But during that period it begin to be ruled by a family, orginally from modern Wales, known as the Corneu.
To futher confuse matters, there was also a Dumnonia and a Cornoaille in Little Britain. One cannot always be sure what place a particular author means, or if the author is simply, rather randomly, using a British name that the author has encountered in other tales with no concern about where such a place might really be.
The most common character associated with Cornwall is King Mark.