Monster Hunter World - A Living Fossil Achievement/Trophy Guide - Capture a fish known as "the living fossil." (Petricanths Rare Fish)

Start an expedition to Rotten Vale Central Camp (11), which is available after unlocking five-star assignments. Once here, you'll want to go to Area 15, and you can find a small blue pond near the end of the trail. Look into this pond, and there is a random chance that a Petricanths will spawn, which is the living fossil fish that we are going after. it's a dark fish with glowing blue spots on it (seen on the video thumbnail).

If it spawns, take out your fishing rod and catch it. Mash B the whole time you're reeling it in, and follow the on-screen directional presses as well. Once caught, the achievement will unlock. If you are having trouble with the fish, you can try using a Baitbug in order to make fishing a little easier.

If the fish does not spawn in this pond, you will want to fast travel back to Astera, and then start your expedition over again to see if it spawns in. Rinse and Repeat until it spawns.