Duncan Targaryen, also called Duncan the Small or the Prince of Dragonflies, is an unseen character in Game of Thrones. He is long dead by the time the series begins.
Duncan was the eldest son and heir of King Aegon V Targaryen. He was named in honor of his father's great friend Ser Duncan the Tall, who had also become Lord Commander of Aegon V's Kingsguard. He died alongside his father and his namesake in the Tragedy at Summerhall.
He is mentioned in his namesake's entry in The Book of Brothers.
In the books
In the A Song of Ice and Fire novels, Duncan was the eldest son of King Aegon V Targaryen, named after his close friend Ser Duncan the Tall. He once defeated the ten-year-old Barristan Selmy in a tourney at Blackhaven. Prince Duncan took pity on the small knight and jousted against him when no one else would, giving Barristan the nickname "the Bold". Years later he was defeated by Ser Barristan at a winter tourney in King's Landing.
Prince Duncan fell in love with a woman known as Jenny of Oldstones and loved her so much he gave up his crown for her and married her against his father's wishes. His love for her inspired a song. He died in the Tragedy at Summerhall alongside his father and his namesake.
Behind the scenes
Due to the simplification of the Targaryen lineage by removing Jaehaerys II from the line of kings and making Aerys and Rhaella the children of Aegon V rather than his grandchildren, Duncan is Aerys and Rhaella's brother in the TV series though he was their uncle in the books.