Hi All, My dog Dingo (best guess - Jack Russell / Basenji mix) is a generally well trained, obedient, and loving dog to people and dogs. I should note that he is my first dog and that I rescued him off of the streets when he was about 8 weeks old off. As a puppy, he would always tend to be overly submissive and roll on his back when meeting new dogs at the dog park. However, now at a little over a year old, he has started to become extremely possessive over toys when other dogs come near him. The toy might not even be "his," but a communal toy at the dog park. He does not show aggression towards people in the same situation, but the hair on his back will stand up and he will growl and even lunge out at other dogs if they simply come near him when he has a toy in his mouth. This is quite a difference in behavior that he is developing and I would greatly appreciate any techniques or tips anyone might have for mitigating this. Thank you for taking the time to read this.