Step 1: Head to Inca Camina province using a helicopter. DON'T travel on land as it will definitely take some time traversing those mountains in the region.
Step 2: Once you're in the province, head towards the river that cuts the very left edge of the map in order to locate the bloodied rock. The exact coordinates for this location are: Latitude= 19.7410; Longitude= -66.6622.
For reference, here's what the bloodied rock looks like:
Step 3: Inspect the bloodied rock and get ready to fight El Yeti after the unskippable animation. Fighting El Yeti is a bit hard as he fires sniper shots in your direction and when you get hit, you will definitely die! So, it's best that you put your game face on in this fight! You can check out the video below by youtube user Carbon Meister for reference of the El Yeti fight.
Step 4: After killing El Yeti, you can climb the mountain to his body and inspect it. After another short animation, you'll unlock the El Yeti ghillie suit for your character to wear.
After that, CONGRATULATIONS! You have finally solved the mystery of the Yeti in Ghost Recon: Wildlands and got yourself a new suit!