Eagle Eye actor Ethan Embry is set to join the cast, while Julian Morris will return as Prince Phillip, Horowitz posted on his Twitter page.
"Ethan Embry coming on #OnceUponATime soonish. #mildholidayspoilers," he posted on Sunday (December 30).
Two days, Horowitz wrote: "Yes, Prince Philip as brilliantly portrayed by @Ju1ianMorris will return... And soonish. #OnceUponATime #holidayspoilers".
Other actors who have been already announced to join the show are Rose McGowan, Gregory Itzin and Chad Michael Collins.
Once Upon a Time will return with new season two episodes on Sunday, January 6 at 8pm ET.
Watch a promo for the return of Once Upon a Time below: