I just found out how to do it. If you completed the game 100% and the achievement won't unlock, you have to load a saved game, then it will unlock automaticaly.
Turning the Xbox on and off as other sites suggested doesn't work...
No.
The War horse code is a code used to unlock the War horse in Red Dead Redemption.
No.
simple internet
You must get to a certain experience level
Go through the story mode and eventually you will unlock it.
you have to rank up to level 50 online 3 times to unlock the zebra
you unlock the first bull at level 50
there is no code you have 2 unlock it yourself :)
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
"DOA" unlock Dead Ops. "ZORK" Unlocks an achievement. "3ARC UNLOCK" unlocks a Zombie map