Sale was a done deal. I agree, seller has to buy pony back now. No backing out legally, anyway … the misrepresentation of buyers motives seems tenuous at best. I would suggest they could start with an arbitrator versus actually going to court. I think it’s much less expensive.
I tend to believe the OP wanted the pony for the niece or whoever it is, she’ll finish it, compete it, trainer and “client” make a nice profit on the flip. I don’t really see the issue here, either, happens every day in the horse business, and the seller seemed pretty motivated to sell since the pony was sold far away without anyone meeting personally, and for a discounted price.