After centuries of wars with the Picts, they united the Scots and Picts under one king, Kenneth MacAlpin, in 843. Kenneth had to incorporate both Picti and Scoti into one nation. He chose to use the Picti saint, Andrew, rather than the Scoti saint, Columbo as the nations saint, and used this to make the new flag. Rather than use either the Picts or the Scots flag, which both had animals on them, he made a whole new flag out of the cross of St. Andrew (an X) and put it on the feild of blue. Then, with Picti saint and Picti based flag, he put in some Scoti and called the country Scotland.
I can not give justice to the History of Scotland by telling you about
it here. There is a book by Donna Fletcher Crow that is the only recommendation
I have so far that does justice to this country's history. Written as a
novel it incorporates the full history of Scotland and the imagination
of the writer together to make a wonderful book to capture the imagination
and attention of all who read it. It's called The Fields of Bannockburn.