I see nothing wrong with having the first right of refusal in a contract. The buyer does not have to agree.
However, this from Wiki:
"Because an ROFR is a contract right, the holder's remedies for breach are typically limited to recovery of damages. In other words, if the owner sells the asset to a third party without offering the holder the opportunity to purchase it first, the holder can then sue the owner for damages but may have a difficult time obtaining a court order to stop or reverse the sale. However, in some cases the option becomes a property right that may be used to invalidate an improper sale."
Right of first refusal - Wikipedia, the free encyclopedia
It is not stupid or immoral to put it into a contract. However, enforcing it will be a challenge. A very old lawyer once told me that, particularly in small claims, "The law is whatever the judge says it is that day in court." There is a lot of truth in that.
Also, unless the horse is very expensive, you would go to small claims and sue for damages, not for the horse. In Arizona, the state bar association has a deal where you call them, and they refer a lawyer who specializes in your area of concern. You pay the bar $35, and the price includes 30 minutes consult with the lawyer they refer. It is a GREAT deal for anyone with a legal question, since you get actual legal advice for $35.
If you hire the lawyer to write a letter, you will probably spend more than you could collect in damages for most horses. I did it for a $15,000 stock in my Mom's estate, and it cost me $700 for the lawyer to write several letters to an out-of-state outfit explaining AZ law and the consequences of ignoring it. For $15,000, it was worth it. But for the value of a disputed horse?