I can certainly understand caring about a horse and making sure it goes to a good home and isn't sold into a bad situation. However, if you choose to sell your horse at fair market value, then that is a risk I feel that a person has to take. You can always lease a horse if you want ownership rights.
The whole selling a horse for fair market value, but signing a contract giving the seller rights is a whole lot of BS if you ask me. I also find it very amusing that a person would sell a horse they care so much about, but wouldn't pay a couple of hundred bucks to a lawyer to draw them up a contract that was actually legal and would stand up in court.
For those of you that think that kind of silly contract is ok when selling a horse then I have a question for you. If a buyer came along and said they would buy your horse except you had to give them a year to own the horse and if for any reason problems occurred then you'd buy the horse back, would you do ti? Of course you wouldn't. To me, it is the same thing.