In the Southwest, Oleander is everywhere. Its evergreen foliage and beautiful flowers make it an attractive landscape plant, used extensively in new housing development.
Unsuspecting newcomers to the area may have oleander planted in close proximity to their horses - or your new neighbors may have planted an oleander hedge right next to your horses' turnout fence line. If this is the case, you may have to resort to offering to pay for and help plant a safer replacement - this will be much less expensive in the long run than the potential veterinary bill (and keep you on better terms with your neighbors - they likely were simply unaware that these plants could harm your horses).
One of the most complete listings available is the Cornell University Poisonous Plants Informational Database which lists plants by both common name and scientific name.
Another excellent resource is A Guide to Plant Poisoning of Animals in North America from IVIS (International Veterinary Information Service), which lists plants by systems affected (helpful if your horse is showing symptoms and you are trying to determine a possible cause).
Accessing IVIS requires free registration.
If you're as bad at identifying plants as I am, you can consult with your area cooperative extension agent to walk your land with you and identify potential problem plants.