The Poisonous Trail Treasure Hunt is one of the many secrets available in RDR 2. You need to find 3 maps and the treasure. This page contains descriptions of all the locations related to this secret - you will locate it without any problems.
The first map
You can start looking for this secret when you reach chapter 2. Head north, to Ambarino. Find a small cabin near Cairn Lake.
Look for the box hidden beneath the bed. Inside, you will find one of the parts of the map.
The second map
The second map is in the south, west from Saint Denis. Head to Lemoyne in Eris Field region.
Look for a withered tree (the one without a crown) with a large hole. Approach the tree and take out the map. This will give you another hint.
The third map
The last map is in the east. This one is hidden in a hole - you can recognize this place by an eye made out of stones. The place is surrounded by pruned shrubs.
Climb the watchtower to inspect the site (just like in the picture above).
Run towards the middle of the eye and search the hole in the ground. Arthur will take out another map.
Treasure - gold bars
The treasure is hidden in a cave located in Annesburg region. The treasure consists of 4 gold bars - thanks to them you will get rich.
The cave is hidden beneath a waterfall. Go through the water, take out the lamp and head further into the cave. Jump down.
Look for a small passage. You can find it on the right side of the wall, in the middle of the stone ledge. Go through it.
Keep following the stone ledge. Go down when you reach the end of it. Keep moving forward until you reach a pile of rocks. The gold bars are inside of that pile. The treasure hunting is over - Arthur is about to become rich.