The lion and the unicorn are actually the symbols of the United Kingdom. They are heraldic devices that appear in the royal arms of the United Kingdom. The unicorn represents Scotland and the lion represents England. This combination dates from the accession in 1603 of James I of England who was already James VI of Scotland. They were used in the coat of arms of Hanover from 1837 to 1866 and also in the coat of arms of Canada since 1921.

1) Nursery Rhyme
The legend that links these two heraldic animals has its own rhyme, numbered 20170 in the Roud Folk Song Index. This one is usually given with the lyrics :
The lion and the unicorn
Were fighting for the crown
The lion beat the unicorn
All around the city.
Some gave them white bread,
And some gave them brown
Others gave them plum cake
And drummed them out of town.
This legend may have been augmented by the Acts of Union of 1707, it is possible that it was a year later that William King (1663-1712) recorded a verse very similar to the first stanza of the modern rhyme scheme, which would seem to have expanded to add more verses.
Besides those above, only one survives:
And when he had beaten him,
He beat him again;
He beat him three times in succession,
His power to maintain.

2) In Popular Culture
Lewis Carroll played this rhyme in “Through the Looking-Glass” where he incorporated the lion and unicorn. In the image above, the crown they are fighting over actually belongs to the white king, however, since they are also on the white side, it makes this rivalry rather absurd.
Carroll displays the classic vision of an alert lion by making him slow and rather stupid, even though he is an excellent fighter. The Unicorn’s role is also reversed by the fact that he sees Alice as a “monster,” even though he promises to start believing in her if she believes in him. Sir John Tenniel’s illustrations for the caricature section of Benjamin Disraeli as the Unicorn, and William Ewart Gladstone as the Lion, allude to the frequent parliamentary battles between the two men, although there is no evidence that this was Carroll’s intention.
An episode from Neil Gaiman’s novel Stardust serves as the basis for the rhyme, in which the novel’s protagonists, Tristran Thorn and Yvaine, witness a fight between a lion and a unicorn over a crown as they travel through an enchanted forest. The accompanying illustration by Charles Vess applies an Art Nouveau style to a realistic depiction of a lion attacking its prey.
