Mistress Gaia wanted to modify this slave's behavior. She did not like how he acted and how he behaved. She had tried to talk to him but he did not change his ways and so she had to change tact. She did so using him as her human pony and making him carry her on his back for hours. It was painful and cruel but it did the trick for the mistress.
Riding ponies is what this mistress is into. He likes riding them all the time but she missed it when she moved to the city. She longed to ride them and when she realized it was becoming a tall order, she got a slave a she turned him into her human horse. She rode him and did all manner of things to him. She humiliated him and enjoyed terrorizing him.