What does it mean if the area between your dog's testicles and anus is swollen and in pain?

Take him to the vet is always the best solution. It could be a swollen anal gland, in which case the vet will need to drain it. It is suggested that you take your dog into the vet A.S.A.P. Yikes, the poor thing is in a lot of pain for goodness sake. It is obvious by the question that this Dog is not neutered. There are various health related matters to concern yourself with unneutered Dogs. Hernias and Tumors of the Testicles and Anus to mention a few. One important concern is the Prostate Gland, and it will enlarge over the course of the Dogs life. As the Dog ages it will likely to become uncomfortable and possibly being large enough to interfere with defecation. The Prostate can become infected also which is almost impossible to clear up without NEUTERING. Neutering causes the prostate to shrink. If this dog is a STUD, or Show quality Registered Purebred, it sounds like retirement for the poor thing. Your only recourse would be to have him neutered. However, if this is your personal pet, which for some reason you have not neutered, should be neutered. Alway's Neuter and Spay your personal companion pet.