Lady Scarlet felt that this guy had to be scared shitless and that is why she used her pony play fetish to deal with him. The mistress was sure that the best way to deal with him was to make him feel pain and be humiliated at the same time. That is why pony play was her preferred mode of punishment. The pony play punishment worked the way it was intended.
Lady Scarlet had tried to warn this guy to stop being a bully but he did not want to stop. So she had no choice but to force him to stop. And she did it painfully as well as humiliatingly. The mistress laughed at the loser as he cried and begged her for mercy but she did not care since she had warned him prior to turning him into a human pony and whipping him.
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.