In Switzerland, it’s illegal to own just one guinea pig. This is because guinea pigs are social animals and they are considered victims of abuse if they are alone

In Denmark too. Also horses are not to be kept alone

We adopted our hound after finding out that our staffy really liked having another dog in the house. She has a therapy dog / sibling.

I'm interested in how this works in terms of pairing.

I have a lone guinea pig. I had two rescue pigs that were rescued from a bad situation, and one sadly died after 6 months. We've had two separate partners, but despite a lot of work in trying to partner them up he's never been able to not fight a potential cage partner.

I want him to have a partner, but the stress for both of being constantly in fight or flight is concerning. We even considered rehoming at a rescue centre and giving a huge donation, but it would just pass the problem on.

This is true, I checked the second to last time I saw this factoid posted, on reddit back then.

Here are my screenshots from the ordinance from that time: https://imgur.com/a/ZkTyEYq

The rule is defined in the Animal Protection Ordinance (AniPO) link at the bottom of this page: https://www.blv.admin.ch/blv/en/home/tiere/tierschutz.html