According to the American Society for the Prevention of Cruelty to Animals, holly berries are toxic to dogs, cats and horses, with a low toxicity. Holly leaves are also toxic.
Holly berry poisoning produces the symptoms of diarrhea, vomiting and depression because of the toxic property of saponins. Hollies come from the Aquifoliaceae family, which includes inkberry, winterberry, European holly, Oregon holly and English holly. Any pet that has consumed or is thought to have consumed holly leaves and berries should be taken immediately to a veterinarian, or call the Poison Control Center to get advice for treating your pet.