This mod allows you to select any horse in the game, (FROM ANY MOD), and give it to any follower in thegame, (FROM ANY MOD). This version provides eight slots for horses, and eight slots for followers. It works with any type of follower, housecarls, hirelings, other vanilla followers, and followers that are standalone or recruited through a multi follower mod. if they are your follower, you can give them a horse that they will ride when you ride your horse. It is not required to buy the horses. You can if you wish, but you don't have to. When the horse is selected, it will not register as stolen. You can give eight followers one horse each, or if you only have a couple followers, there is the option to give them each four horses. etc... The horses will follow at a distance when player is on foot, and fast travel if they are not being ridden. If they are Being ridden during fast travel, they usually show up a short time later, or can be forced to show up immediately upon a second fast travel to the same place, or by using the games wait feature for one hour. Once the slots are filled, they cannot be changed, (future feature perhaps). You can uninstall the mod, do a clean save and reinstall the mod to start over with a clean slate. Considering the short amount of time it takes to select and give a horse away, (a few seconds),this shouldn't be an issue. There are  only four scripts in the game. One to start the quest, one to select  a horse, one to give the horse away, and one to summon the horses, (they may spread out during combat or just wander away further than you'd like.) The horses have a follow package and a stay put package. They will remain stationary when not being ridden, and start following after a certain distance from the player is reached. This distance is roughly equal to the distance between say,  Whiterun Stables and Honningbrew Meadery, maybe a quarter mile I guess. There are only four steps to get started. 
 They are as follows:
Step 1  Go to Whiterun Stables and pick up the three spell tomes near the barrels in the right most stall.
Step 2  Learn the Set Horse spell, the Give Horse spell, and the Summon Horses spell.
Step 3  Find the horse you want a certain follower to own and select it using the Set Horse spell.
Step 4  Give it to your follower by using the Give Horse spell.
That's all there is to it.  Personally, I like to use the last slot for my own horse, so I can summon it with the rest of the herd.

Just a note, don't let your followers see you ride their horses, it could produce undesired results.
(Unless losing your follower or reloading after dying is not important to you)

The horses used in the videos are from Swift Steed Stables.
The followers used in the videos are from Cortex Follower Series, (and Lydia obviously).

I can't thank mitchalek enough for his Convenient Horses mod which was the main inspiration for this endeavor.
It will always be in my load order. (Its compatible) Thank you for your inspiration and encouragement.

Thanks to djjohnjarvis for Sofia - The Funny Fully Voiced Follower, which inspired the process used to assign the horses.
Again, always in my load order. (Its compatible, but unnecessary since Sofia has her own Set Sofia Horse Spell)

Of course many thanks go to Bethesda for the game and the Creation Kit to make mods possible.

And lastly but not least, thanks to Nexus for this platform, still the best in my opinion.