Not too long ago on here, willows, ash trees, and honeylocust trees were mentioned as being on the safe list for horses. Globe willows will thrive in just about any conditions. They grow fast and provide a lot of shade.
Thanks you guys for the info. Have any of you heard how Fruitless Mulberry trees do up here in the North West? My mom has one in her back yard but she lives in CA so not sure how it would do with the winters here. Makes a great shade tree though.
Fruitless mulberry (Morus alba) is listed as Sunset zone 7, which covers most of western Oregon and Washington, but probably not Idaho nor east of the Cascades. However, microclimates vary, and if the location is sheltered it may work for you. Check to see if any regional nurseries (do an internet search) carry it...it will be hardier if grown nearby.
I had Maple in mind but wanted to make sure it would harm the horses if they ate the leaves and such. Maple sounds like the best bet and I like them. I was going to stay away from the pine trees because of the sap, needles, & pitch. Thank you everyone for your suggestions and info!
We really like Norway Maples (not the Reds). They are tough, horse-safe, and lovely (beautiful gold in the fall). They do have seeds, though we've not had a problem with sprouts. Male (seedless) ash, hybrid poplars (not the spindly ditch bank ones), European Hornbeams, and Lindens (both the large-leaf and the small leaf) are some other favorites. All are hardy, and all except the ash and the poplars are fairly disease and insect resistant. London Planetrees are great, if you don't mind the mess (little prickly seedballs, great for keeping kitties out of flower beds.) The native elms that grow wild around here are good too, but they seed and sprout like wildfire. Nursery grown cultivars of the Silver Maple are beautiful, hardy and fast growing, but may become brittle under snow and ice if not regularly pruned properly. Of all of these, our favorites are the Linden and the Norway Maple.
For evergreens around here, the Austrian pines, Colorado and Norway Spruce do quite well, but my critters seem to like munching on those more than any other trees. I'll admit, a pony with piney breath really isn't so bad...
Back when I owned and landscaping nursery, I wrote a pamphlet called "Landscaping for Livestock". I'll see if I can't round up an old copy and post it. If you'd like pictures of any of these trees, feel free to drop me an email.