It's illegal to own only one guinea pig in Switzerland. Because they can get severely depressed when all alone.
Denying your pet the chance to make friends with members of its species could be considered an act of cruelty at least in Switzerland. According to Techly, an animal rights law introduced there in 2008 makes it illegal to own just one guinea pig at a time.
The law was part of a legislative push to grant "social rights" to pets that tend to get lonely. Guinea pigs, for example, are herd animals, so having at least one furry companion to share a cage with should supposedly boost their quality of life.
Because it's unlikely that two guinea pigs will die at the exact same time, pet owners can find themselves in a legally sticky situation when one of their animals passes away. Fortunately, there are now rent-a-guinea pig services in the country that provide partners to live with lonesome guinea pigs for the remainder of their lives.
There are other pets in Switzerland that are forbidden from living in isolation. Goldfish are also required to have at least one tank-mate and parrots must either live or have the opportunity to socialize with other birds on a regular basis. And though most cats may seem like solitary creatures to their owners, their social wellbeing is also protected in Switzerland; the law doesn't go so far as to say that pet owners must have more than one cat, just that single cats should be able to see other cats when prowling outside or staring through a window.