Poisonous Plants in AZ
Angel Trumpet/Devil Trumpet /Datura
Devil's Trumpet (also known as Jimsonweed,, Angel Trumpet, Datura stramonium) is highly poisonous to horses in Arizona and everywhere else, containing tropane alkaloids (atropine, scopolamine) that cause dilated pupils, rapid heart rate, agitation, tremors, convulsions, coma, and potentially death, so you must keep horses away from it.
Fiddleneck
Fiddleneck (Amsinckia Intermedia) contains some potent pyrrolizidine alkaloids that can cause severe liver damage. It may take some time to kill or injure the horse but can happen quite quickly depending on the dose.