General information for livestock
The University of Illinois Veterinary Medicine Library has a good database of plants toxic to animals (including photos and detailed description of the effects of the plants on animals): https://guides.library.illinois.edu/plantstoxictoanimals
Cornell University has a poisonous plants information database, which is arranged by type of animal, which covers horses, cattle, goats, sheep, poultry and pigs: http://poisonousplants.ansci.cornell.edu/anispecies.html
Horses
RIRDC has published a guide for horse owners to help them prevent plant poisoning in their horses: Plants Poisonous to Horses: an Australian Field Guide by Melissa Offord. You can download a PDF copy of the guide or order a hard copy from this link: http://www.horsecouncil.org.au/wp-content/uploads/2015/11/Plants-Poisonous-to-Horses-Aust-field-guide.pdf
The US animal welfare organisation ASPCA runs a national poisons control centre which includes a list of the most frequently encountered plants that have been reported as having adverse effects on horses (it also provides information on pets): https://www.aspca.org/pet-care/animal-poison-control
