Two Celtic-speaking peoples, the Picts and Gaels, founded the Kingdom of Scotland (or Alba) in the 9th century.
Later, the neighbouring Celtic-speaking Cumbrians, as well as Germanic-speaking Anglo-Saxons and Norse, became incorporated into the Scottish nation. 
The Aberlemno Serpent Stone:
Pictish symbols depicting serpent, double disc and Z-rod, and the mirror and comb:
: As recorded on Wikipedia (accessed 11-9-2017): Bede used a Latin form of the word Scots as the name of the Gaels of Dál Riata.Roger Collins, Judith McClure; Beda el Venerable, Bede (1999). The Ecclesiastical History of the English People: The Greater Chronicle; Bede’s Letter to Egbert. Oxford University Press. p. 386. ISBN.
: Pictish stone photo: Catfish Jim and the soapdish at English Wikipedia, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported: https://commons.wikimedia.org/wiki/File:Serpent_stone.JPG