Here’s an article that discusses Florida law in relation to agency and equine transactions. While Florida has very robust codified and common law on the subject, that doesn’t mean there isn’t something wherever the OP is (if not Florida) that could be used here. Some of the actions discussed are fraud, unfair competition / deceptive trade practices, and it’s important to note that fraud or breach of fiduciary duty can include material omissions (failure to disclose) as well as active misrepresentations. Florida also has specific rules on dual agency. What is interesting about Florida is that if there is any monetary harm, that is enough to find unfair and deceptive trade practice. A few other states are also discussed.
https://www.floridabar.org/the-flori…ts-and-others/
While it might be possible for the sale from you to trainer to be voidable because of fraud, and therefore the subsequent sale was invalid and you can get the pony back, most likely your remedy is going to be getting paid damages for what you would have gotten had trainer told you of the other party’s interest and closed the deal appropriately without coercing you into accepting a bargain price and then marking up the pony and who knows what kind of double dipping on the commissions.