This achievement is easy to hunt on MISSION OF SAN JUAN. you'll encounter some rare animal like jaguar and wolf when playing story.. so maybe your progression toward this achievement will be 2 or 3 OUT OF 5. So go to Mission Of San Juan and from outside entrance gate, run through the wooden bridge, then take right down the path and jump to other side... you'll find wolf cave and here you can grind for this achievement.. don't forget to LOOT.. if you're full on animal hides, then sell it on nearby shop to clear some space. then kill it and loot..
(Note : i don't know exactly... killing brown wolf is counting towards achievement or not.. But i am sure that black wolf is rare animal and you can encounter it Both MISSION OF SAN JUAN and top left of THE HIDDEN CITY)