Please tell me you have something in writing? Or it was written on the ad at least?
Horse Dealers are out to get money. I was stung by a woman before on a pony who tried to double barrel my daughter in the head. Because I had collected the pony I let the woman know under no circumstances was I keeping the pony and let her know I would be on her doorstep in no time.
You can try and threaten legal action, say you have the conversation recorded, ect. But you'd be chancing your arm and likely be told where to put your legal action as he'd call your bluff.