Prince Aegon (not to be confused with his younger cousin/nephew, Aegon, the son of Rhaenyra and Daemon) is the eldest son of King Viserys and his second wife, Alicent Hightower. Even after the birth of Aegon and his siblings, Viserys still plans on making Rhaenyra, his daughter from his first marriage, his heir. This frustrates Alicent and her faction at court, though viewers learned in "House of the Dragon" episode nine that Aegon had mostly accepted that his father didn't want him to be king. Aegon, too, is a dragonrider, forming a bond with the golden-scaled dragon Sunfyre.
After Viserys's death, Alicent, her father, and their allies faction seize power, crowning him Aegon II, all before his half-sister Rhaenyra is even notified of her father's death. Rhaenyra isn't willing to give up her claim so easily, and the entire Targaryen clan and their allies throughout Westeros take sides in what becomes a brutal civil war.