Even though we had dogs my whole life, we always had bitches, the first male dog we had was my lovely rottie Billy. After I had him neutered, I came home one day and whilst greeting him, noticed he had two ball-shapes on either side of his willy. I was very concerned and took him back to the vet and said that I thought they'd missed his testicles and somehow they'd slipped down. I can still feel how my faced burned as the vet explained, in great detail, how dogs get erections and that they are one of the few mammals who use muscle tissue to get an erection, as opposed to increased blood flow and that the lumps were not, in fact, left behind testicles. Oh the shame!