Most toxic plant for a horse?

There are many plants that are toxic to horses, too many to post here. For a good list of plants that are toxic (bad) for horses, visit the ASPCA website. Click the Animal Poison Control link and then go to the Toxic and Non-Toxic Plants. Click horses and you will be provided with a good listing of plants that are not good for horsers. Plus you can click on non-toxic and find what plants can be safely planted around areas where horses are being kept.