I remember seeing a prothesis of a testicle that was used in a Halter horse that only had one dropped. They inserted it into scrotom of this stud horse so they could show him and it not count against him and people would still breed to him. I wish I could remember his name, but he was a big name horse as this man showed all over the US and often had the top mare and stallion in points in the US. But I am not interested in Halter horses, just wanted to share that bit of fooling the public and the Assn.
If the one does not drop, two things go wrong . He will pass that trait to the male colts. Probably pass that trait to the mares as she will pass it to the stud colts. So, people need to know this that want to breed to this stud. Then the other part that is aggravating, is when they go to casterate him.....make sure you know your vet well as I have heard of vets telling people that they got both when they did not. And often that retained testicle can be up as high as a mare's ovaries. But if you take the one out, and leave the retained one in, he will not be fertile as the one retained testicle is not air cooled like the other one that hangs low. If that testicle is hot inside, he cannot produce sperm. But, he will still think he is a stallion if it is left in. So he will be hot to trot when you ride him around mares and such.
Sometimes they want to charge a lot to go inside the horse fishing for that other testicle and say they cannot find it. Some vets will not mess with it. They make you carry your horse to where they charge you more than you want to pay for an average horse. Hopefully your horse will drop and things will go smooth. But maybe you can find a vet that will not charge you for the price of a new used truck to dit it out.
This is hard to say when writing down......When a foal is being formed inside the mare, the deciding factor if it will be a mare or a stallion is the male and female hormones. The ovaries develop if it is a mare, and if it is the stud, then what was ovaries in a mare, they become testicles and they drop slowly as the stud colt grows.
If that is worded where it doesn't make sense, I apologize.
So that testicle is between where the ovaries are and his cod sack.
In bulls and I have a feeling that it is the same in horses. The larger the testicles are, it means he is more fertile and this is a trait you go by picking cows out of this bull. He can easily pass that trait to the mare which means her ovaries are well formed and more fertile.
So his cod sack is important.