Join Date: Jul 2015
Location: Western Massachusetts
If you know you'll have to fence them off from the horses, you don't need to worry about the criterion of "safe for horses". Except for avoiding well-known very toxic plants like oleander, why not just look for desert shade trees that thrive in your area?
I would check out Sunset Western Garden Book for nearly complete listings and descriptions of trees for your specific zone, which it will also map for you. SWGB is the bible for gardening west of the Rockies.
Short horse lover