His original name at birth was probably Nero Claudius Drusus after his father, and the agnomen "Germanicus" was added to his full name in 9 BC when it was posthumously awarded to his father in honour of his victories in Germania.
His original name at birth was either Nero Claudius Drusus after his father, or Tiberius Claudius Nero after his uncle, the second Roman emperor Tiberius. The agnomen "Germanicus" was added to his full name in 9 BC when it was posthumously awarded to his father in honour of his victories in Germania.