Guinea pigs are herbivores, and their diet primarily consists of plants. Cucumbers are a good source of vitamins and minerals for guinea pigs, and the skin is also edible. However, it is important to wash the cucumber thoroughly before feeding it to your guinea pig, as it may contain pesticides or other chemicals.
In addition to cucumber skin, guinea pigs can also eat other parts of the cucumber, including the flesh and the seeds. However, it is important to feed guinea pigs cucumbers in moderation, as too much cucumber can cause diarrhea.