Look at at this way. Cats and dogs hunt and eat small furry mammals. 'Nuff said!
So, if you currently own a cat or dog, think twice about getting guinea pigs. It can work if you have sufficient indoor and outdoor space that the cats and dogs don’t have access to. But even if Puss and Fido are the friendliest guys on the planet, the guinea pigs will still be extremely nervous. The sight, sound, and smell of the carnivores will spook them, even if the cat and dog are just trying to be friendly.
Guinea pigs are spooked by the mere presence of cats and dogs
Guinea pigs can eventually accept very docile cats and dogs, but any interaction needs to be supervised. The GPs move very fast, and a cat or dog's chase instincts can easily kick in, even if things have been going well. A paw placed on a guinea pig to stop it running could seriously harm your guinea pig, physically and mentally. It really is best to just keep the species away from each other.
So, if you own a cat or a dog, you'll need a warm garden shed or indoor room for the GPs that your cat or dog has no access to. A cat who simply sits by a hutch and stares can cause untold stress to the guinea pigs. If you want to make sure your GPs stay happy and healthy, keep the big guys with the claws and teeth well away!