Hi there. Sadly that easy and straight forward approach doe snot work. Using a summon creature type spell and marking the horse as summonable somehow impacts the horses AI and Functionality. The horse will not be rideable and follow your around.
You will need to use a script type spell and attach a script that spawns the horse by PlaceActorAtMe() and makes the player the horses owner. There is a "Summon Ethereal Horse" Mod on the Steam workshop: Get it, extract the scripts in the bsa and look how it's done there.