In former times, the king and queen of France's oldest son was called the dauphin.
dauphin in British English
(ˈdɔːfɪn, dɔːˈfɪn, French dofɛ̃)
noun
(1349–1830) the title of the direct heir to the French throne; the eldest son of the king of France
Word origin
C15: from Old French: originally a family name; adopted as a title by the Counts ofVienne and later by the French crown princes
dauphin in American English
(ˈdɔfɪn; ˈdoʊfɪn; French doʊˈfɛ̃)
noun
the eldest son of the king of France: a title used from 1349 to 1830
Word origin
Fr, lit., dolphin: used as a proper name by the counts of Vienne, and hence as a title by the oldest son of the king after the province of Dauphiné was ceded to the crown