Ok, I've had it with invincible Shadowmere ruining my game by charging into battle...is there a mod available where I can make my horse just stand there after I dismount? If it means the horse just stands there being beaten to death, that's ok. Playing on legendary level and Shadowmere's health bar never even flinches, even against ancient/legend dragons..

Kuertee's Horse Commands
http://skyrim.nexusmods.com/mods/15225

You set your horses to run away or be aggressive. It can be set on horse to horse basis.