My 6 1/2 month old GSD Eva has a couple times now stolen another dogs toy and when the dog tries to get it back from her she growls and snaps a little. She's also done it with a stick she will have found on the ground. She doesnt do it with every dog in the park. Some she will completely ignore or submit to. If there is no toy or stick to guard then she plays happily with any dog . She took a toy off a JRT this morning and the dog was chasing her trying to get it back, Eva was growling and snapping. I told Eva to drop the toy, which she did and I then gave it back to the owner. She did this a couple of times and again I did the same. Then the owner put the toy away and Eva and the JRT happily played together again. Eva has no issues with dropping a toy for me or my kids. Shes not food aggressive. I'm going to speak to the trainers at dog classes tonight but I thought I would ask here aswell for advice on stopping this dominance over toys/possessions in the dog park.