A number of plants are poisonous to guinea pigs, including bracken, bryony, buttercup, charlock, deadly nightshade, foxglove, hellebore, hemlock, lily of the valley, mayweed, monkshood, privet, ragwort, rhubarb, speedwell, toadflax and wild celery. Additionally, any plant which grows from a bulb (e.g., tulip and onion) is normally considered poisonous.
I believe in animal rights, and high among them is the right to the gentle stroke of a human hand.