In Assassin’s Creed Valhalla, there is a glitch where if you save and log out while you’re sitting on a horse, then close the game, there will be a second horse when you restart the game and load your save. This horse will usually look identical to the first, though not always.
Source: N4G PC In Assassins Creed Valhalla, There Is a Glitch That Lets You Duplicate Horses