Yes you can give guinea pigs the occasional apple. This should not comprise a large amount of the diet as they are hind gut fermentors---their gastrointestinal tract is not structured to digest the sugars found in fruits. The diet should mainly consist of timothy hay and a pelleted guinea pig diet. Supplement with fresh green vegetables. You will also need to give oral Vitamin C tablets. Water should be available at all times. Feed treats sparingly.
