I posted a question a few weeks ago about my lilac point siamese boy, Chino, only having one testicle. Since then he has been neutered but it was major surgery. The vet needed to open his abdomen to search for the missing testicle! Apparently it can be life threatening if a testicle is left behind because it becomes cancerous.
After 90 minutes in surgery the vet couldn't find anything and has suggested it could be a congenital fault that he only has one testicle. Even so, I need to keep a close eye on him over the next few weeks. If he displays any male cat tendancies it means the rogue testicle is lurking somewhere and producing hormones. In which case it's back for more intensive surgery.
I'm glad to say Chino is recovering well, albeit feeling very sorry for himself wearing a big plastic collar to stop him biting at his stitches.
Here he is with his half sister Coco, a little seal point siamese.