Lady Scarlet felt that her slave was not scared enough of her. She felt that she had to do something in order for him to know that she was serious in her quest to make him get scared of her. She was not satisfied with being respected. She wanted to be feared as she felt that that would make him do anything and everything that she asked him to do.
Lady Scarlet has a thing for ponies but she could not have one due to housing restrictions where she lived. So she resorted to turn her slave into a human pony so that she could enjoy having one even if it was not a real one. She made him strip naked and she whipped him to get him to cooperate with her and do the things she wanted him to do.