As everyone has said, there is definitely a market. Trainers and parents need nice ponies for their kids. Trainers and parents do not want the semi-feral ones fresh out of the breeder’s pasture, or the super green ones right off the plane from Europe. If you can bridge that gap by finding a quality pony, developing the pony correctly, putting a good-riding kid on it for videos and shows, and representing it well, you will have interest.
If you really want to do nice larges, be aware that you’re starting right off in a saturated, competitive section of the pony market. If you aren’t already experienced in selling horses and don’t have a great lead right now on a quality large, I’d suggest starting off on smaller scale with one that will be more of a children’s or even a local show type, use that to test your process, and build from there to a division prospect.
I have landed on a solid niche within ponies that I am optimistic about, but it did take me a year of working on connections and research, plus a starter sale pony, to figure out. Along the way I found another pretty good pony niche in my area which a friend of mine is now pursuing. You might be surprised by what you figure out once you get going.