They would be cavy ear tags. I worry about ripping, the cold and the metal tag, will they stay in, etc. I am also curious if they could be used as wing bands on poultry.
Ear tags for cavies are required for show. I've had a few with ear tags, never seemed to bother them. BUT I always kept my piggies inside too. Never had a problem with them getting ripped or causing issues. They were designed for cavies, I'd stick to the plastic/metal feet bands for poultry.