Had the same with a lurcher, a big lad, would run at a dog (only ever big dogs though) would not even check to see if it were a dog or bitch, young or old would just suddenly REALLY lay into it, seen him drag another lurcher his size across a gravel path on its side.
He was a perfect dog till he reached maturity around the 2yr mark, prior had never bothered other dogs, never been in a fight, just wanted to walk by my side. I even took him to the vets, got the drug which replicated the effects of castration, did no good, contemplated putting him to sleep.....his aggression had escalated to at