[F4M] Trained into being your ponygirl
(Bonus if you have read the story 'Izzy's Ponygirl Adventure', as this is (very) loosely based on that)
It's about 500 years after the last goblin was eradicated. Technology has barely progressed since then as it took years to recover from the war that happened.
You are an old, fat man on the outskirts of our small town. Most people know that you don't want to be bothered, your research always much too important. Nobody really knew what it was that you do out there, but it was clear that you came from a lot of money.
I am a desperate girl that can't find a job in the town. Everyone knows that my parents were called to a war by a distant lord, and it's been 4 years since they returned. Nobody says it to my face, but everyone knows they aren't coming back. It's been hard for me, trying to afford life. For a while, I just lived off of what they left, and a small stipend since they were fighting in a war. But now I have come to age, and I don't receive those benefits anymore. After searching for a few months to find work, it was a last ditch effort to go to your manor on the outskirts of town.
What I didn't know is that you have found an ancient goblin artifact, one that was placed on ponygirls. (I was thinking a collar for this, but I am fine with other items as well!) It's imbued with ancient goblin magic, making it so anyone wearing the collar isn't able to run away from you no matter what. Also, when you are getting on the saddle on my back, it shrinks you down to the size of a goblin (but keeps all your weight) allowing you to ride on my back if you like. Maybe it has another effect as well, so that with training I'm only able to release my pee with your command.
Obviously, this is a very broad idea. I am looking for a slow training prompt filled with humiliation, degradation, punishments, etc. I am very open to ideas, so please message me if you have any!