Celtic contributions to Arthurian history were examined and found to be perhaps difficult to substantiate. Pictish connections to Arthurian history are a bit easier to find.
The Picts were a group of people so named because the Romans called them that. The word Pict came from the Roman word for painted people. As with so many things in this part of British history, we have to see the beginnings at least through the eyes of the Romans.
To Rome, the Picts were a nuisance, continually jumping the northern barricades and marauding throughout the Romanized British countryside. The Picts were not exactly British; rather, they were their own breed, to themselves answerable and for themselves only accountable. They resisted the Roman occupation with fierce determination and, hindsight would say, only appeared to give in to Agricola, all the time waiting for the opportunity to raid southward again. Most of all, they wanted to be left alone.
They had built up quite a good little Celtic society in what we now call Scotland; the Romans wouldnt let them keep it.
Enough about Romans. What of Arthur? We know that Arthur had trouble with the Picts. We know his predecessor High Kings had trouble as well. Vortigern was deathly afraid of Pictish raiders and took the extraordinary step of inviting the Saxon leaders Hengist and Horsa to settle in Kent in exchange for their help in stemming the tide of Pictish gains against Vortigerns kingdom. Vortigern, it will be recalled, is intertwined in the pages of history with Merlin, no matter what his incarnation. (Vortigern it was who employed the young Merlin to interpret a vision of red and white dragons that Merlin said predicted the eventual Saxon victory.)
Arthur, too, would have had trouble with Picts. An examination of possible sites for Nenniuss famous 12 battles of Arthur tends to reveal the possibility that some or many of the 12 battles took place far in the north, perhaps in Scotland itself. Could some or many of these battles have been against the Picts and not the Saxons? Its possible. Nennius says the battles were against the Saxons, but he doesnt exactly say that all 12 were against the Saxons. The Roman word for the Pictish land was Caledonia. One of Nenniuss battles was said to have taken place in the Celidon Forest. Many historians believe that Celidon was really Caledonia. This battle is also said by some writers to have been the cause of Merlins madness.
The land of Lothian, from whence Lot came and Morgause went, was in the far north, perhaps a Pictish kingdom. The existence of Lot in Arthurian history is considered to be factual. The Arthurian characters of Gawain, Gareth, etc. are from the Orkney Island, even farther in the north. Did the Pictish connection extend to the Orkneys as well? Perhaps.
It was in the Picts fierce fighting spirit, however, that we find the closest link to Arthur. Nenniuss descriptions of Arthurs battles (their inflated victory totals notwithstanding) do justice to the warrior image we have of Arthur: He had no mercy for the Saxons, who were trying to take away his kingdom; and he had not time for delaying actions or diplomacy. Certainly mindful of the treachery that befell Vortigern (Hengists slaying of British nobles at a supposed peace conference), Arthur resolved to take the sword to the Saxons. Pictish drive, if not blood, carried him through it.