Julian Cope presents Head Heritage

The source of the Brutus foundation myth was Nennius , a Welshman , then Geoffrey of Monmouth ,another Welshman built on that . I think you may be thinking of Scota ,the Scots equivalent . Foundation myths like over imaginative (hi)stories are everywhere .Another one is the Scots genocide of the Picts , resulting in a unified nation .
The King list , like the above was written centuries after the most of the kings had died . Considering that the Picts had no written records they were unlikely to be accurate , they are false up to just before the the time of writing , that is different from being falsified which suggests some tampering of the “facts “ . The use of gaelic was part of the assimilation of that language and culture into that of the Picts ,it can be seen in material culture where the earliest Pictish stones had no christian iconography by the mid period around the time when Kenneth was crowned , Christianity the religion in the Gaeltachd , informed and was mixed with Pictish symbols , by the end of the tradition the Pictish stones had a only christian symbols ,a clear transition rather than abrupt revolutionary change .