All you half to do is ride a horse majority of the time. There is no specific location. And it may take some time to because i found the horse after i got the last horse "death".
To get your dead eye level higher you must complete the story mode missions until you are able to get to the next level of dead eye.
Kill other players, complete gang hideouts and challenges, etc.
Do your missions.
The four horseman are Death, War, Pestilance, and Famine.
Level 50, or Legend 1.
do the missions which it says in your journal
Level 40.
Complete gang hideouts.
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.
There is no such thing as Red Dead Redemption black there is only the regular Red Dead Redemption and Red Dead Redemption limited edition.
You unlock the mount Zebra at the highest level in multiplayer, which is 4th Legendary level 50.
When you reach the 4th Legendary level 50, you can then get it!