Red dead redemption 2 strange horse problem [closed]

Note that when you load a save, you're not resuming the game from when you saved. By that I mean that Arthur is always shown to be doing something such as leaning against a post, sitting on a rock, ...

Additionally, the game sometimes automates a few things for you. Most notable, it auto-equips additional weapons when you leave your horse when your current mission will contain a shootout (you can use this to predict ambushes), or putting your horse in an easily accessible location (nearby, near allies' horses, ...)

It's thus not farfetched to assume that the game intentionally gives you one of your horses when loading in, as it assume that you might need your horse.

First in stable all of a sudden there was extra horse.

If you're referring to the first stable visit (mission with Hosea), there is a second horse but it didn't come out of nowhere.
During the game, the first horse you receive is in the Adler ranch (where you rescue Sadie). This is your horse, but when you start the Hosea stable mission, Hosea gives you a second horse (and tells you to sell it).

When you enter the stables, your first horse is stabled there, and you ride in on the second horse.

Map said Arthur's horse, but all previous horses were in the stable. Just little weird. Just some bug?

This could be a horse you've ridden which became a temporary horse. Actual ownership of a horse happens by moving your saddle, but when riding a horse you don't own, it will after some time become "yours" in the sense that it listen to your whistles as you're roaming around.
This horse also persists in the game world, it will stay where you've left it. There can only be one temporary horse - when another horse becomes your temporary horse, the earlier horse is unregistered as yours.

So it's possible that when you loaded, the game spawned your temporary horse near you, while keeping your owned horses in the stables.