Unfortunately I unknowingly purchased a horse with a history in this situation.
I didn't ride her at the vetting, and later realised that she was probably doped when I tried her. Having discovered the horse's full history, i was lucky to just get away with a couple of weeks on crutches!
any legal action is against the person you paid the funds to, rather than the agent.
I was totally stitched up and would never buy from anyone acting as an agent.
For the record, the mare was found a suitable, permanent home. She was too unpredictable to sell or breed from.
I brought a horse from a dealer, a year and a half later i decided to part with him () and resold him through the dealer. In other words she sold him in behalf of me and the lady who brought him wasn't happy with him.She was given my contact details and rang me and told me this whole sob story (sounded like a complete novice out of her depth) however i didn't write the advert or provide the photo's the dealer did. So when the new owner told me i had to buy him back (wasn't in a position to unfortunatly) i told her to speak to the dealer as there wasn't anything i could do. The dealer then had him back and happily rehomed him to someone else.
In terms of legally if the horse is not as described then i believe you have 6 months in which you can return him. Therefore either the dealer or owner should take the horse back. It all depends on how solid the contract is with the dealer.
This 'Selling on behalf of a client' idea has escalated over the last few years as you lose many of your rights as it is not the dealer's animal so you only have the rights you would have in a private sale. I firmly believe that many of them do belong to the dealer but it's a way to evade responsibilities.
When you buy through a dealer acting as agent for a private seller, it is legally the same as making a contract with the seller directly. Any representations made about the horse's suitability by the dealer would apply as if they had been made by the private seller in a private sale.
This means that you have all the rights to cancel the contract under the standard laws of misrepresentation if the animal is fundamentally misrepresented, but as the dealer is not a party to the contract, you do not get the rights you would have if you purchased from the dealer (a commercial seller) under the Sale of Goods Act etc.