In most jurisdictions, that's criminal behavior, a type of fraud, and it's criminal behavior in the horse business, too, but it's extremely rare for anyone to bring charges. If the seller is honest, you can defend yourself against this kind of thing just by asking him whether the purchase price includes any commissions. If he says he's paying your advisor a commission and you didn't know about it, you know your advisor is a thief. But if the seller is dishonest and is engaging in simple bribery, he won't admit it, and you'll have no way of knowing that it happened. Many sellers pay honest commissions to honest brokers, so there's nothing at all wrong with commissions, as such.
Undisclosed commissions are the problem.
At Public Auction Since more horses are sold at public auction than privately, there are more opportunities for monkey business there. Again, the kickback is king, and it works the same way, except that you're in a better position to defend yourself. What you're defending yourself against is a scheme that typically is initiated by your advisor's asking the seller what price would make him happy. Then your advisor tells the seller that he'll guarantee that price if the seller will kick back to him everything above that figure that you bid. More likely, he isn't a professional and is just helping you out because he's a nice fellow who wants to assist someone new to the business.