I noticed that some hunters have horse AI packages...and that's pretty cool but I was curious how to make an NPC who spawns use a SPECIFIC horse, or like how can you specify what horse a NPC uses. I dont see where the information for it is, does anyone know?

Anyone?

NPCs will, I think, only use horses they or their faction have ownership rights to, so you can manipulate the ownership of a horse to determine who will ride it.

Will they instantly jump on the horse just because its nearby though? Like how do I make sure that the NPC is going to use the horse while they travel around?

I want to make Imperial Legions patrol on horseback for example.

I've only dealt with horses in the context of followers, but from what I have seen, if the "Rides Horses" checkbox in the appropriate AI procedure (travel, patrol, etc.) is set to true and there is an available and owned horse nearby, an actor will move to it and mount it as soon as their AI tells them to move.

I haven't looked into this, but http://www.creationkit.com/SetVehicle_-_Actor might work (as said on the wiki)