Sebastian chews on his leash too! I use a harness and I was using a retractable leash but, he chewed right threw it. We weren't even walking we were sitting out in the front yard with the kids. Next thing I knew he was free!! I just went and boust a regular leash thinking that he wouldn't chew on that and he does. If I leave his harness on him he will try to chew through that. I think he just likes having something to chew on. I may try to get some bitter apple to spray on it and see if that helps.
