Wow! Even Zeus "Champion Toy Destroyer" hasn't managed, yet, to destroy the black Kong - that's the toughest of all the Kongs. Though in all honesty he doesn't seem to like the taste or maybe it's the smell - I find Kongs have a strange smell, which to me anyway, isn't all that pleasant.
Have you tried to Black Kong? The only other indestructible toy I know off is the Aussie Staffy Ball but I wouldn't recommend it as it's so hard it caused injury to Ra Kismet's face, quite a nasty gash, and also I got hit with it in the shin and ankle and limped around for a couple of days. I was actually nervous of getting hit in the face with that ball as it could do serious damage, like a broken nose/teeth etc. And it's quite expensive - almost AU$100.00 it's now just an expensive white elephant.