The Pictish king-lists are virtually the only source we have for these. The many problems of interpreting these names are covered below. However, in the north, there is fragmentary evidence of names that do not appear to be of Celtic origin. Additionally, there are Ogham inscriptions from the north that include names and name formulas that are consistant with those in the Pictish king-lists, but that are otherwise indecipherable. For the sake of accuracy, this should be acknowledged, but from a practical viewpoint, there is no reason not to lump all the non-Celtic material into one consideration.
Some writers describe Pictish as a Celtic language with an admixture of some non-Celtic substrate.
In some cases, the influence may have been on the actual names themselves, but in many it can be demonstrated to be a later scribal artifact only affecting the recorded forms, similar to the recording of many vernacular names in Latinized forms. It is certain that whatever non-Celtic element existed in Pictish eventually disappeared. What is in question is whether that element had been reduced to mere vocabulary items in a Brythonic matrix by the time of the earliest records or whether non-Celtic Pictish was still a viable language at that point, although with significant Brythonic borrowings. Basque is the only remnant of this type surviving today, although there are early records of others, such as Etruscan, that did not survive. For this reason, some writers have tried to relate Pictish to Basque directly.