According to the University of Arkansas, Lilac Syringa Vulgaris is low on the scale for toxicity to livestock, and the plants are generally considered non-toxic to humans (http://www.aragriculture.org/horticulture/ornamentals/toxic_plants.htm). While lilac bushes are not considered toxic to livestock, it is wise to keep horses away from the bushes and do not feed them clippings.
Persian lilac (Melia azerdarach) which is not related to true lilacs, is poisonous to dogs and horses, according to the ASPCA Animal Poison Control Center. Other names for Persian lilac include Chinaberry tree, the bead tree and Pride-of-India.
It is advisable to keep horses away from lilacs and ensure that they have good quality forage. For more information on toxic plants, go to: http://www.extension.org/pages/33853/what-plants-are-toxic-to-horses. There is also a good Michigan State University Extension bulletin on ornamental plants that are toxic to horses at: http://web3.canr.msu.edu/horseadults/publications/Toxic%20Plant%20Bulletins/Ornamental.pdf.