How can you give units horses for upgrades? This Upgrade Requires Horse Mount & Blade II Bannerlord Guide talks about the different unit upgrades that require horses and what you must do in order to upgrade these units to the next tier, introducing mounted combat as the units new specialty.
You can only directly equip horses in two ways. You can equip a horse to yourself to use in combat and exploration and you can equip one to a Companion. You cannot give basic units, or equip them with, a horse of any type. If a unit requires a horse to upgrade, you need to have a specific horse in your inventory. This is consumed when the unit is upgraded, giving them the horse as a mount for their new tier of gear.
Upgrade Requires Horse Mount & Blade II Bannerlord GuideI’m unsure if it’s the same across all factions but the Khazuit Nomads, for example, require the Desert Horse to upgrade into the next tier when it says “Upgrade requires horse”. It’s very likely other factions have similar restrictions but without playing them all, I cannot be certain. So if you’re looking to upgrade the Nomad units, check for the desert horses.
Certain horses do have types. The cheaper, more basic horses, are just no type. The more advanced, more specialized horses are Horses and War Horses. So, if you’re playing a different faction, just head to a nearby town, save the game, and buy one of each horse to see which horse is required for the upgrade. That should be enough to upgrade any units that require a horse.