Little buggers 
I gave my GP1/GP2 hot water bottles put inside some zip up waterproof pillow protectors (mainly in case the unlikely chance of them bursting/leaking)
I bought Pet Hotties too (softer than Snugglepads with a cover) but they didn't keep warm and lickily the pigs didn't eat them.
I'm read a post years ago - someoes pig ate a bead so it was a 'What has your guinea eaten' thread.
Mine used to eat shoelaces with the plastic bit
Any fabric
Books
There was one poster who's pig ate all the rubber buttons from the TV remote 
It's a thing with them.