Cats and guinea pigs have very different survival instincts and because of this they don't always mix well. Cats are predatory animals and guinea pigs are prey animals. Even when a cat appears to just be playing with a guinea pig it can inflict serious injury in an instant. Therefore, before you even bring a guinea pig into your home it's imperative that you determine how you will keep the two away from each other.
Unlike many dogs, cats are agile jumpers and climbers and can often get themselves up onto high surfaces with little effort. While a child gate could be effective in keeping a dog away from your guinea pig, that type of barrier won't redirect a cat. Therefore one of the best ways to ensure that your cat and cavy don't interact is to keep your piggy in a separate room behind a closed door when you are not there to supervise your cat's actions.
Put a Lid on It
Making sure your guinea pig's cage has a sturdy and secure lid on it can help reduce the chances that your cat will be able to inflict any harm. Making a homemade cage out of coated-wire grid cubes, which are sold in stores as storage units, and Coroplast, a plastic poster board, will allow you to attach a secure lid to the enclosure. The small openings in the grids will prevent your cat from even being able to place his paw inside the habitat.
Cats are inquisitive by nature so it might be beneficial to introduce your cat to your guinea pig when you first bring him home in a safe manner. If the mystery surrounding the new cavy is eliminated, your cat may lose interest quickly and may not pursue future interactions. Holding your guinea pig firmly in your hands against your torso while you let your cat sniff him can make for a good introduction.
As prey animals, the guinea pigs' most predominant survival instinct is that of flight. When they sense danger they immediately seek shelter. Having a place where your guinea pig can hide when your cat is near his habitat will make him feel more secure and will decrease the chances that the two can interact. Pet stores sell various shelters designed specifically for cavies made from either wood or plastic.