You must keep doing the sotry mode missions until you find a mission labelled as "S" on the map, this is a Seth mission.
The War horse code is a code used to unlock the War horse in Red Dead Redemption.
Yes, there is a mission in which you do this.
No.
If you want to be able to do this you need to complete the story missions on Red Dead Redemption and eventually you will unlock the higher levels of dead eye which allow you to use the X's.
You unlock them as you level up through the red dead redemption multiplayer
There is no such thing as Red Dead Redemption black there is only the regular Red Dead Redemption and Red Dead Redemption limited edition.
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You must get to a certain experience level
There are two, Red Dead Revolver, and Red Dead Redemption and Red Dead Redemption Undead Nightmare
you unlock the first bull at level 50
there is no code you have 2 unlock it yourself :)
Go through the story mode and eventually you will unlock it.