Possibly, but it would be bad for them. Horses have been known to eat them, frequently with fatal results.

Nerium Oleander is one of the most poisonous plants. The bright colored flowers & sweet fragrance are very alluring, but it's loaded with cardiac glycosides which will quickly stop the heart.

Nerium (oleander) is listed as a 'deer resistant' plant, meaning they don't usually eat it, but may do so if they're starving.