In House of the Dragon S01E01 "The Heirs of the Dragon", King Viserys Targaryen had a conversation with his wife, Aemma Arryn, where he compared his brother, Daemon Targaryen to Visenya Targaryen, one of the wives of his ancestor, King Aegon I Targaryen (the first king to sit on the Iron Throne):
Aemma: Rhaenyra has already declared that she is to have a sister.
Aemma: She even named her.
Viserys: Dare I ask?
Aemma: She chose a dragon's egg for the cradle that she said reminded her of Vhagar.
Viserys: Gods be good. This family already has its Visenya.
Aemma: Has there been any word from your dear brother?
What did Viserys mean by this? In what way is Daemon Targaryen the "Visenya" of the family?
Note: I'm assuming Viserys was referring to Daemon, based on the conversation. If I'm mistaken, my question should still stand – in what way is that family member, a "Visenya"?