Princess Serena sits on his back because she has a great desire to ride him. Her slave pony has to carry her around on his back and do everything she commands him to do. Then Princess Serena puts her legs over his shoulders and notices that her smelly sneakers are very close to his nose in this position. So does it smell nice and intense?! Her human pony gets her extreme foot scent right up his nose - and Princess Serena just loves it! The more unpleasant a situation is for her slave the more she enjoys it. That's why she takes off her smelly sneakers and ties a shoe around her slave pony's head. With his nose deep in Princess Serena's smelly shoe, he now has to carry her around and let her ride him!