historical-nonfiction:

Dauphin (dolphin in French) was a sort of pet name that the son of the Count of Vienne, Guigues, was given by his English mother when he was a child. When Guigues inherited the title and lands of the Count of Vienne, he formally called himself “The Dauphin,” and had a dolphin as his motif on his arms. The territory he ruled became known as the “Dauphiné.” When it was later purchased by the King of France, he gave the land, and the title, to his son and heir. Which is why to this day thee heir to the throne in France is called the “dauphin.”