The object/animal/person in this episode is Cora's heartless victims rising from the ground.
The scene with David and all the mirrors is a nod Gaston Leroux's famous novel "le fantome de l'opera" - The Phantom of The Opera- where Raoul is trapped in a maze of mirrors to get to his lover. It this case it is the same, David is trying to get to Mary Margaret so they can reunite each other.
Colin O'Donoghue (Captain Hook) and Sarah Bolger (Princess Aurora) previously worked together in The Tudors (2007). Sarah Bolger played series regular, Princess Mary (future Mary I), and Colin O'Donoghue made a one episode guest appearance in The Undoing of Cromwell (2009) playing Duke Phillip of Bavaria. Ironically, in that episode of The Tudors, Phillip was a potential husband for Mary and they even share a kiss during the episode.