Ask Mr. Smarty Plants

Because horses are famously adept at finding ways to hurt themselves or make themselves sick, we're always careful how we answer questions like yours.  Even seemly innocuous objects can harm horses in certain situations. 

In addition to protecting your horses from your trees, you should give some consideration to protecting your newly-planted trees from your horses until the trees are large enough to withstand the rubbing and chewing that will very likely occur.

First, it's important to know what you very definitely do not want in your pasture or along your fencelines.  Keep no species of cherries or plums (Prunus spp.) anywhere near your pastures.  In certain circumstances the foliage of these trees can be fatally toxic to horses and other livestock.  Avoid most nut-bearing trees as they can cause colic and sometimes other maladies.  Red maple (Acer rubrum) should be avoided due to toxicity.  Though it's doubtful that Black Locust (Robinia pseudoacacia) grows in your area, it should also be kept out of your pastures.

Pines (Pinus spp.) are generally safe and fast-growing as is American Elm (Ulmus americana).  However, it would be a very good idea to ask your equine vet about any guidance he or she can give you.  You might also talk to other horse-owners in your area for their recommendations.  Finally, Equisearch has published some excellent guidelines that might be helpful.