@vjb - Sadly, I don't know of anything a pen might be made out of which would defeat a determined dog's jaws. Remember they're designed to reduce anything remotely edible to small chunks, and that includes bone... I had a narrow escape when our eldest Golden, Teg, borrowed a Parker '51' blue diamond and wandered off with it. Fortunately I was able to retrieve it before she settled down for a good chomp, and her soft mouth meant it was unmarked at that point!
Honestly, if you really liked the Lamy 2000 there aren't many pens which would be tougher. Makrolon is pretty strong stuff, but clearly not as strong as tooth enamel powered by bone-crunching jaw muscles! I'd even say don't go for a metal pen, because even those would be made of fairly thin metal to avoid being too heavy, and thus crunchable. Something made of titanium might survive... but would likely be very expensive, and any surface coatings (lacquers, chrome etc) would probably be shredded. Our mob will happily play with metal objects and will even chew stones.
If you really want a metal pen then something like a Waterman Perspective might be a good choice, but it's a c/c filler and being made of solid brass it's quite heavy. I have an old style Rotring 600 which might survive - it's been dropped in the road and run over by a car and survived - but again it's pretty heavy and they're no longer made.
I think the best solution is probably to get another Lamy 2000 and guard it with your life! :)
