If England is represented by the lion, the unicorn represents her northern neighbor, Scotland. Visit Scotland reports that the unicorn first appeared in Scottish national heraldry in the 12th century, when William of Northumberland became William I, King of Scots. (Ironically, William was born on the English side of the Anglo-Scottish border, and was known as "William the Lion," according to Britannica.) Later, the unicorn device became so closely identified with the Scottish kings that Scottish coins featured a unicorn on one side.
When Elizabeth I died without an heir, her nearest Protestant kin was James Stuart, then James VI of Scotland. Parliament invited him to London to reign as James I of England. He accepted the new role, but brought his unicorn imagery with him, decreeing that England's royal coat of arms (seen above) should now feature the English lion on the left and the Scottish unicorn on the right, to show the equality of the two nations under his reign.
Why, exactly, did he choose the unicorn? True, other nations had adopted animal imagery — a lion for England, an eagle for Spain, and so on. As the National Museums Scotland succinctly explains, "In Scotland, James I went for the unicorn. We don't really know why."
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