Back in the summer of 2015, the co-directors of Disney’s “Zootopia” went rogue, disregarding the advice of their corporate colleagues and appearing onstage at the Annecy animation festival disguised as the talking-animal cartoon’s fur-covered lead characters, an anthropomorphic rabbit and her foxy best friend. The audience ate it up, but behind the scenes, the Disney suits were sweating. For decades, the family entertainment company had been dealing with a very specific group of enthusiasts, identified as “furries,” who get off on dressing up in full-body animal costumes, drawing inspiration wherever they can find it. By now, Disney lawyers know how...