J.k. Rowling said this quote

“ Harry flung himself out from behind the bush and pulled out his wand.EXPECTO PATRONUM! he yelled.And out of the end of his wand burst, not a shapeless cloud of mist, but a blinding, dazzling, silver animal. He screwed up his eyes, trying to see what it was. It looked like a horse. It was galloping silently away from him, across the black surface of the lake. He saw it lower its head and charge at the swarming dementors. . . . Now it was galloping around and around the black shapes on the ground, and the dementors were falling back, scattering, retreating into the darkness. . . . They were gone. The Patronus turned. It was cantering back toward Harry across the still surface of the water. It wasn't a horse. It wasn't a unicorn, either. It was a stag. It was shining brightly as the moon above. . . it was coming back to him. ”