How do I start the treasure hunting challenges?
The first treasure map can be found during an ambient challenge, happening at any time and at any place. Lookout for a man on the side of the road being robbed by bandits, help to save him and you will obtain the first map.
Where can I find the Four Horses of the Apocalypse?
The four horses will not spawn near roads. Watch for a message to let you know a mythical creature is nearby.
Pestilence spawns in the north
War spawns in the frontier
Famine spawns in Nuevo
Paraiso and Death is awarded after the final mission in the main story.
Death can be found in any of the apocalypse horse locations before the end of the game as long as the player has broken the other three.
What happens if one of these horses die?
If one of the Four Horses of the Apocalypse dies or is replaced, you'll only be able to access it again after completing all of the Four Horses of the Apocalypse challenges. Then you'll receive the deeds for all four horses.
How do I get the deeds to these horses?
If you have not completed all of the Four Horses challenges but you have complete the story, you'll be granted the deed to Death automatically, you can still earn all deeds by finishing the challenges.
How do you find Sam Odessa and complete 'California'?
First complete the 'Women and Cattle' mission. This is a prerequisite.
Then find Sam Odessa in the stranger mission 'California' by looking for a question mark on your map just south of the 'h' in Gaptooth Breach.
A few days later, look for the second question mark icon just north of Cueva Seca. The third question mark is up along the road through a cut in the hills near Gaptooth Ridge. The fourth and final area is just south of Tumbleweed. Take the letter and mail it from Blackwater to unlock Treasure Hunter Outfit Scrap 2.
Remember that it will take a few days between each encounter to find each new question mark. Advance time by saving repeatedly or by playing through each day. Entering and exiting the save screen advances time by six hours, even if the user doesn't complete the save. This can save a little time when done repeatedly compared with actually going through the save process each time.
Where can I find beavers?
Beavers have eluded many players. Check along the water west of the Pacific Union Railroad Camp and north of Beecher's Hope. Stand still and look for movement in the grass. Be careful - for a full listing of just about every animal location, see earlier guide.
Where is the Chupacabra in Undead Nightmare?
The best chance of finding the Chupacabra is in Nuevo Paraiso on the last tier of the hunting challenges. Look around the fields northwest of Torquemada, southeast of Perdido, and south of Casa Madrugada. You'll know it's nearby when you see a message and the radius of its possible location on the map.
From RockStar official blog...