Fan CastingBatman And Owlman: Under The Red Hood

Description of "The Mutant" (Post-Credits Scene):

Upon returning to their home universe, Batman and Owlman part ways with Doctor Fate, with the crisis on two Earths having finally been resolved. But when they return to Wayne Manor, they are immediately greeted by Dick Grayson, who hands Bruce a roll of newspaper and says, "Take a look at this." and Bruce reads the paper, which reveals that Doctor Kirk Langstrom, one of the doctors in Arkham Asylum, went missing, and the last hint the crew of the prison got pointing towards his absence was that of a Bat-like mutant creature shedding Kirk's Arkham attire. The eyes of both Bruce and Thomas, Jr. widen, knowing full well what they'll be up against now.