I mentioned in a previous post that acorns are quite poisonous to horses. There are quite a few other trees which are - plums, peaches, cherries, etc, as well as scarlet maple and sycamore maple.
What kind of trees do we plant around horses? Obviously, it depends on the location, but willows make good shade trees. Weeping willows are particularly good because the trailing branches are kind of fun for animals to play with and around. Ash trees are another good choice, as are magnolias. Birch and lime are popular in England.