My little mutt Kiki became very aggressive after he was neutered. But then again, he as already two and a half years old when he was fixed. When we adopted him, he was friendly with other dogs and strangers, now, he's a nightmare There are only about two or three dogs he's friendly with, and some people he likes. I'm not sure why this happened after he was neutered though When on walks, I walk him on a chocker/martingale(half check) collar. When he starts barking, I give him a sharp tug on the lead, and he stops(only for a short while though. Lol). It just doesn't seem to click with Kiki.