Idaho is an open range state and livestock have the right of way. However, open range doesn't apply in fenced areas and this happened in a fenced area. If the driver sues the horse's owner, he has a good chance of winning. If it was outside a fenced area, the horse's owner would collect the value of the horse.
Moose have the right of way no matter where they are.

“In a time of deceit telling the truth is a revolutionary act.”
― George Orwell

It's not over when you lose. It's over when you quit.