Previously, the Fox X-Men Universe and the Marvel Cinematic Universe were totally distinct.
The following post and videos contain SPOILERS for Episodes 1 through 5 of WandaVision.
“On a Very Special Episode…” featured the return of long-dead member of the Marvel Cinematic Universe: Wanda’s brother Pietro, better known as the Avenger Quicksilver. Only this wasn’t the Quicksilver from Avengers: Age of Ultron.
This was Evan Peters, who played the character in three X-Men movies for 20th Century Fox.