Yes, there are poisonous chestnuts. While the chestnuts that we typically roast and eat during the holidays are not poisonous, there is a type of chestnut that is. The horse chestnut, also known as buckeye, contains a toxin called saponin. This toxin can cause vomiting, diarrhea, and even death if consumed in large enough quantities. If you suspect that your dog or cat has eaten a horse chestnut, call your veterinarian immediately.