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 foxglove you planted, you will have more problems keeping it over the winter to see next year’s flowers than with it taking over the garden. Grecian foxglove (Digitalis lanata) is very invasive and should not be planted. Most of the others reseed moderately and often winter kill. Depending on where you live, it can be difficult to get the plants to their second year when they bloom. Winters without snow cover make keeping the plant difficult.