Yeah, every state is different. Think it has since been removed from the books now but no more then 20 years ago in one state, a motorist was required to stop, dismantle their car and carry it past a horse if it spooked.
In Oregon everything yields to horses but you wont find that in our drivers book either. Unfortunately it is buried in the regs but where you find the signs are on state park multi use trails. You might try your states park services and see if they can direct you to the proper regulation.