Mistletoe could lead to heart problems, but most of the time it just causes a severe upset stomach.
Holly is usually not a problem but can cause oral irritation within 24 hours.
Christmas trees, whether cedar, pine, etc. may cause simple digestive upset if excessive amounts are taken in.
A Christmas cactus is very common around the home and is not harmful to your pet.
Daylilies and especially tiger lilies that are used in holiday bouquets can cause kidney failure in cats from just a few bites.
As for treatments, for every 10 pounds of your pet's weight, give 1 teaspoon of Kao Pectate to soothe an upset stomach. For excess ingestion - CALL YOUR VET! Most of these plants are usually eaten by dogs and are not dangerous unless the pet consumes too much.
Poinsettias are the most asked about holiday plant veterinarians are called about. The truth is, there is no actual toxicity associated with any part of this popular holiday flower.