Ragwort is common in colder places and a lot of oak leaves and acorns
Also heard that if a horse eats too much Ferns it can be poisonous, but isn't really the most toxic plant compared to what horsetail can do to your horse for example.
Exactly what kareen said. Some really common ones are acorns, and horsetail.
That's already a good thing to know the name, but the most important is being able to recognize them to avoid your horse to eat them...
Here you can get more information:
There're so many... Eggplant, avocado leaves, cherry, chive, red maple ...