Only infrequently will a horse die suddenly from plant toxicity. Death can result from ingestion of cardiac glycosides or cyanide-containing plants. Plants that cause death within a couple hours include yew, poison hemlock, spotted hemlock, or European hemlock and death camas. Avocado peels should never be fed to horses nor should horses be pastured next to avocado trees. An unknown toxin derived from this plant causes death following episodes of colic after a couple days. Avocados are deadly to all large animal species, and goats die within 48 hrs.
For more information, read Toxic Plants.