In the second episode of "House of the Dragon," Prince Daemon steals a dragon's egg from the Dragonpit, claiming he wants it to lay in his son's crib. Princess Rhaenyra is particularly outraged, since she chose that egg for her baby brother who died in the first episode. We don't know yet which dragon will hatch from that egg, but we do know some major info about the dragon who laid it: Dreamfyre, who will almost certainly feature in the show.
Dreamfyre hatched sometime during the reign of Aegon the Conqueror. Her main rider is Aegon's granddaughter Princess Rhaena. In Martin's books, Dreamfyre lays many nests of dragon's eggs, some of which are stolen and never returned.
After Rhaena dies, Dreamfyre makes her home in the Dragonpit, where she remains until Princess Helaena Targaryen (daughter of Viserys and Alicent) bonds with her. We didn't see Dreamfyre in season one, but we'll presumably see Helaena ride her in season two.
