Cherry plants and fallen leaves aren’t safe for them either
Creeping indigo is highly toxic to horses. I lost my last mare to it.
Also horsetail and I THINK the least deadly is buttercup
The ones that are tasty and very deadly are oleander and yew- they work quickly and taste good to the horses. But there's also rhododendrons, nightshades, etc
Oak trees(acorns), bracken, yew, sycamore, ragwort, buttercups
It's a good question... Unfortunately, they are numerous. For the safety of horses and especially their health it is better to know them all. The most toxic species for the horse are:
Great Absinthe, Aconite, Adonis, Anemones, Arnica, Belladonna, Male Fern (Rhizome), Ellébores, leaves, fruits, stems of If, intoxicating weed, Pure oleanders, Laurier-Cerise, Black nightshade, Horsetail, Rhododendron, leaves and dried fruits of Ciguës, Rue (Mediterranean basin), Sycamore maple, Sabine (Faux Cyprès ), The leaves of Buis (Buxus sempervirens), the Semen-Contra, the Ergot of Rye, Vesce (Vicia sativa), the Vératre of the mountains, Thuya, Tobacco, Colchique (Autumn Narcissus, Safran Bâtard) , The Jacobean Sénecon, the Euphorbes, the Ferula, the Poppies, the Datura Stramoine, the Digitales.