Well he's 8 years old and I guess he was gelded when he was young and never had the other one removed. I'm not really sure. But his current owner, when she got him, he mounted a mare and did his deed, so she called a vet and asked, and I guess that's when they realized he only had one testicle.
The tried removing it, but like I said, just as they were going to, he sucked it back inside.
He's not really stalliony or tries to mount any mares (he's in a pasture with 1 mare and 1 gelding, and there have been no babies.