Aslan

"Have a care, Rabadash," said Aslan quietly. "The doom is nearer now: it is at the door: it has lifted the latch."

"Let the skies fall," shrieked Rabadash. "Let the earth gape! Let blood and fire obliterate the world! But be sure I will never desist till I have dragged to my palace by her hair the barbarian queen, the daughter of dogs, the——"

"The hour has struck," said Aslan: and Rabadash saw, to his supreme horror, that everyone had begun to laugh.

They couldn't help it. Rabadash had been wagging his ears all the time and as soon as Aslan said, "The hour has struck!" the ears began to change. They grew longer and more pointed and soon were covered with grey hair. And while everyone was wondering where they had seen ears like that before, Rabadash's face began to change too. It grew longer, and thicker at the top and larger eyed, and the nose sank back into the face (or else the face swelled out and became all nose) and there was hair all over it. And his arms grew longer and came down in front of him till his hands were resting on the ground: only they weren't hands, now, they were hoofs. And he was standing on all fours, and his clothes disappeared, and everyone laughed louder and louder (because they couldn't help it) for now what had been Rabadash was simply and unmistakably, a donkey. The terrible thing was that his human speech lasted just a moment longer than his human shape, so that when he realised the change that was coming over him, he screamed out:

"Oh, not a Donkey! Mercy! If it were even a horse—even a horse—e'en—a—hor—eeh—auh, eeh-auh." And so the words died away into a donkey's bray.

Chapter 15, Rabadash the ridiculous. page 199, The horse and his boy.