Why Wanda just became the Scarlet Witch
Jimmy Woo confirmed on WandaVision that Wanda did not yet have a fun superhero nickname. Viewers know her as the Scarlet Witch, but she’s never been called that on screen until the end of episode eight after Agatha got the full picture of her powers.
She has usually been portrayed as a mutant, like the X-Men, and started out as a supervillain alongside her brother, Quicksilver, but then she switched sides to join the Avengers. At one point she did have magical twin sons, and those sons turned out to be part demon, courtesy of the demon supervillain Mephisto. In later comics, she’s no longer a mutant but was born a witch.
In the MCU, the Scarlet Witch appears to be more of a concept than a specific person, at least based on how Agatha gave Wanda this title. Wanda had visions of a witch when she first touched the Mind Stone during the HYDRA experiments, and Agatha gave her the name after learning how Wanda created her sitcom world and made Vision completely out of magic.