Monster Hunter World: Iceborne is filled with all sorts of things to do, and a whole lot of creatures to discover. Many of the Achievements, such as the Golden Gleam Achievement, involve finding a very rare creature.
Monster Hunter World: Iceborne – Golden Gleam Achievement Guide
The creature you need for the Golden Gleam Achievement can be found during the day, on Hoarfrost Reach. It is an extremely rare spawn, so you will need to be vigilant and patient to find it. It will spawn with other similar creatures around a hot spring in the area. You will be looking for the Goldspring Macaque.
If you have trouble finding the hot spring, it is in Area 2, just above Area 17. The hot spring itself is direct across from the number 2 on the in-game map. It is a good idea to leave your little Palico at the camp for this one, as the creature is skittish, and might get scared away by your small furry friend.
Frankly, the Golden Gleam looks like a cross between a monkey and a raccoon, and the spring is not so large that spotting him should be difficult when he spawns. Just keep a good distance and use your binoculars to check if he is there.
Because the little guy is so skittish, it is a good idea to use Ghille Mantle, and wait for him to turn away to approach. When he does, just run up and grab him.
I really cannot express enough how patient you need to be to get some of these Achievements, so don’t despair if it feels like it takes forever for this guy to show up, because it might take that long. Best of luck!