Can you actually stable your horses or is it for carriage purposes only? I haven't picked up a steward yet but Frost and Shadowmere could use a house too.

Carriage and stables are separate.

Edit:
So, to elaborate, you can use it as a home for your horses.

How do I get them in the actual stable? I took shadowmere to my house and s/he just stands at the loading point.

Ride them into the stable, turn so that they're almost sideways inside (you have to do this to dismount).

They've stayed inside for me, as long as they aren't being attacked.


Edit:
Unfortunately, if you fast travel, they still come with you.

If you have Dawnguard, park your horse in the stable, walk off and go summon Arvak and ride him, your horse will stay in the stable permanently. One of my characters had Shadowmere and another horse he owns at Riften stables for ages. Shadowmere did not like sharing a stable at one of the new houses though, so it seems the new horse there might be moved to another house. They could have made the stables a bit bigger.

If your horse is Shadowmere, just park him in the stable, then kill him. 15 days later, he will respawn in the stable and ready to fight invaders away. I don't use Shadowmere as a horse, I use him as an unkillable sentry to defend my home from giants. Even if he dies, he will respawn.

I tried to park Shadowmere in the stable, it really is small as aussie500 mentioned. I normally use Arvak but yeah, Shadowmere decided not to like the stable and stood outside after I left home and came back lol. Good idea on using as a sentry, I might just move Frost back to Riften for safe keeping. I've had random draugr corpses on my land? And been attacked by a group of bandits so far. Also found out that my housecarl is a mage yet he wears scaled armor

the stables are nice but Ive noticed too that they are very very tiny and dismounting in the stable is pretty much impossible. You can but parking to fit the horse in there undernieth the roof can take up to around 7 mins.

even then after zoning and return to the stable the horse/horses do not stay benieth the stable roof and just stand outside.

nice stable but too small or too low of a roof.

***note: falkreath / lakeview stable does not have a lantern. Other 2 locations do. Not a big deal but location wise in the forest there it can get very dark at night to even see if your horse is still there at night or if its dead due to bandits, giants, vampire attack, dragon attack.

not to derail but want to add that maniquins still spawn anywhere after placing armor on them. Meaning they do not stay in place and spawn even in the door ways.

I figured the stables were automatic, for storing extra horses. I am disappointed by that feature.

I haven't had problems with my maniquins in Windstad manor but in Honeyside, they would always bug out. I've even seen them walking around in my basemant. CREEPY.