Origins of "The lion and the unicorn" in British history
The Lion and the Unicorn lyrics date from 1603 when King James VI of Scotland became James I of England unifying the Scottish and English kingdoms. The 'Virgin Queen' Elizabeth 1 named the son of Mary Queen of Scots, James, as her heir. The union of the two countries required a new royal coat of arms combining those of England which featured two lions, and Scotland whose coat of arms featured two Unicorns hence "The lion and the unicorn". A compromise was made thus the British coat of arms has one Lion and one Unicorn and the poem about hence "The Lion and the Unicorn" was created.

The Lion and the Unicorn poem

The lion and the unicorn were fighting for the crown
The lion beat the unicorn all around the town.
Some gave them white bread, and some gave them brown;
Some gave them plum cake and drummed them out of town.

The Lion and the Unicorn poem