Reportedly one bite will not kill a horse, but it is toxic and can cause serious medical issues, including death. I've linked info below that says the ingestion of roughly 30 leaves could kill a horse.

Unless they have developed oleander varieties that are more cold-tolerant than oleanders have been historically, oleanders are short lived here because they are reliably cold-hardy only to zone 8. I grew them in zone 8 Fort Worth, and an occasional winter that was colder than usual killed them every time--or about once every 4 or 5 years.

If the oleander is on your property, as long as the horse cannot reach it, ever, under any conditions, I wouldn't be unduly worried about it. However, if your horse is an escape artist, I'd get rid of the oleander if I were you.

Dawn

Here is a link that might be useful: Oleander and Horses

Thanks for the info....very helpful!