UrsulaPandress sorry to hear about the cruciate ligament. It is definitely multi-factoial and has a lot to do with the angle and alignment of the hindlimb which is where (early) neutering and its effect on long bone lengthening comes in. I'm sure I'm not the first to tell you that the cruciate ligament in the other stifle are equally prone to rupture. Fingers firmly crossed that Bastardspaniel avoids that.
I don't know that there is the same weight of evidence wrt cons of castration in the horse; horses very rarely develop cruciate ligament issues or indeed any of the list of conditions I mentioned in dogs.
I think it's very much a management issue; certainly there is a very big difference in the temperament and behaviour of even the best-schooled stallion vs a gelding. I have a fair bit of experience in this as I'm an equine stud vet (who owns a setter).
I would say that virtually all male horses not specifically intended for breeding should be castrated between one and two years of age. Leaving them to two is ideal imo but entirely depends on temperament and management. A 500kg stallion has frightening power and drive.
Going back to the dogs, I think that a lot of very early neutering, as practiced by many "rescues", is being shown to be harmful though well-intentioned. Hormones have systemic effects on the brain and body which are not entirely understood and whipping out gonads is not just like clipping toenails.
It's interesting to look at practices in Norway and Sweden, where neutering of dogs is uncommon but responsible dog ownership is taken far more seriously.
Well, that's that thread nicely derailed! Sorry OP! 