Monster Hunter World - A Living Fossil Trophy / Achievement Guide - Capture a fish known as "the living fossil." [Silver / 30G]

For this trophy / achievement you must capture a "Petricanths". It's a rare glowing fish. You must use the fishing rod to catch it. It can spawn in the bright blue waters on Rotten Vale map. It can spawn at any daytime and on any rank/difficulty. See the video for what it looks like & the location.
It's an extremely rare spawn and will not show up on each visit. Your best bet is to fast travel to Ancient Forest map and come back. I recommend Ancient Forest because the creature needed for trophy "Snuggles for All" spawns there, so you can try your luck with different trophies at the same time.

Furthermore, the creature needed for trophy "Bristles for All" spawns in the same location as the Petricanths fish. That other creature will spawn on the back of the nearby mosswine. So go to the mosswine first before it walks away and then check for the Petricanths fish. If it's not here fast travel to ancient forest to look for the third creature and come back.
It took me 1,5 hours of fast traveling to find the fish.

Side Note: Fast traveling back to camp does not make fishes/animals respawn. You must travel to a completely different map to reset everything.