Princess Serena missed riding ponies and since she did not want to go in search of them, she chose to use a human one. She used her slave as a human pony especially since he had messed up. She had a reason to humiliate him and she did not worry about how he felt since he had messed up and she was having fun and punishing him at the same time.
Mistress Jamie Kate did not like her neighbor much. She thought he was a loser and she treated him as such. She called him to her house today because she was bored and she needed something to do to occupy herself. She made her lick her bare feet before she turned him into her human pony. She rode him not caring how heavy she was and how he she was hurting him.
Mistress Alina was bored and that was when she found use for this guy who had been stalking her. She let him come to her house and she turned him into her human pony. It helped that she was wearing a bikini so he thought she was interested in him. But he was turned into a pony and she rode him for hours and even whipped him when he strayed.