Foxglove is poisonous to mammals. See the link to the Cornell University Web site at http://www.ansci.cornell.edu/plants/php/plants.php?action=display&ispecies=horses for more information on poisonous plants. You can follow the links in the Cornell reference to the Canadian database as well. There was no information on how much a horse must eat to become sick. It does have to be ingested to make a horse sick. You should consult your veterinarian for more details. All parts of the plant are poisonous. Depending on which …