The characters in the nursery rhyme 'I had a little nut tree' are believed to refer to the visit of the Royal House of Spain to King Henry VII's English court in 1506.
The 'King of Spain's daughter' refers to the daughter of King Ferdinand and Queen Isabella of Spain.
There were two daughters, Princess Juana and her sister Katherine of Aragon. The princess in the nursery rhyme is probably Katherine of Aragon who was betrothed to Prince Arthur, the heir to the throne of England. Arthur died and Katherine eventually married King Henry VIII.
It was sad that "So fair a princess" had such a difficult life with Henry as she was the first of Henry's six wives and discarded by the King to make way for Anne Boleyn.
Queen Katherine was much loved by the British people who hated her replacement, who they called 'The Great Whore'. The young, beautiful princess relates to the young Katherine, as a princess and is immortalised in this old nursery rhyme.