When you buy a horse from a merchant or get proper ownership through other means, that horse will be attached to your character and you may call your mount at your own discretion, but be careful because if it dies, you'll need to procure another.
How it works
A horse in Aurbis is a property, an animal, and a piece of your travel logistics. You don't summon a copy; you call your horse, the one you own, by name. Hitched, it stays where you left it. Tracked, you find it on your map. Named, others learn it by name too. The market in good Nordic ponies is real because horses die — to a sword stroke, to a long winter, to a slip on bad terrain — and you cannot conjure another.
Commands — /help horse
/horse — Show active horse status.
/horse list — List account horses.
/horse info — Inspect the active horse.
/horse track — Find your horse.
/horse hitch — Hitch the active horse.
/horse name <name> — Rename the active horse.