It's the first-known operation of its kind on an endangered species.
Veterinarians say the surgery was performed on a Przewalski horse named Minnesota.
The horses are native to China and Mongolia and were declared extinct in the wild in 1970.
One of the surgeons who helped with the horse surgery pioneered reversible vasectomies in 13,000 humans.
He calls this case an "interesting turnaround," because humans actually became the "test animal."
The 20-year-old horse had a vasectomy in 1999.
Experts say a vasectomy may be performed on an endangered animal because of space constraints, the size of the species or if an animal has already produced many offspring.
