Sounds like a worm. Certain types of worms can be very long (whipworms I think). It is common for animals to still pass worms after they've been de-wormed and I believe there are also worms that aren't killed by your typical de-wormer. I would call your vet and decribe it and then they can give him either more de-wormer or the right kind. No worries, but it definitely needs to be taken care of because if there are any live ones in there, they can wreak havoc on puppy's system. Good luck!
__________________  "You become responsible forever for what you have tamed." - Antoine de Saint-Exupery |