MONSTER HUNTER: WORLD

This achievement is awarded for capturing the Bristly Crake, a legendary rare bird known to ride the back of Mosswines. I would only recommend going for this after unlocking five-star assignments.

Once you're ready to go for this achievement, start by going to an item box, and equipping the Ghillie Mantle, which is extremely important for this.

Next, start an expedition to Rotten Vale Southeast Camp (1). Once here, talk to your Palico, and make sure to leave them behind. Then, put on your ghillie mantle, and proceed down the path, jumping down to the left and following along the left wall until you reach a tunnel entrance that has a Mosswine. There is a random chance that the creature we're after will be on the back of this Mosswine.

If it's not, fast travel to Rotten Vale Central Camp (11) and you'll want to go to Area 15. On the opposite side of where you enter, you should spot another Mosswine with another possible chance at spawning the rare bird.

If it does spawn in one of these two chances, take out your capture net, and capture it. Once caught, the achievement will unlock.

If the bird does not spawn at all, you will want to fast travel back to Astera, and then start your expedition process over again to see if it spawns in. Rinse and Repeat until it's available to be captured. The bird is pretty rare, so it could take you a while before finding it, but the entire loop described above, shouldn't take you longer than 2 minutes and 30 seconds.

You can see the video for further information.

Start an expedition to Rotten Vale (Central camp). From here go to the position in the video and you will find a very small blue pond. Above this small blue pond is a small path with a pig and some stuff to collect. The pig is the host for the stiff, bristly creature, a bird. So check carefully the path and the pig before you leaving the area and try again. If the bird there, normally it "flys" away to the end of the path (this location see you in my video) and there you can catch it with your capture net.

Unfortunaley it is luck if the bird there or not. Is the bird not here, leave Rotten Vale and travel to a other area like Ancient Forest and come back. (Camp to Camp travel in Rotten Vale does not work!)

Honestly I personal needed over four hour that the creature appeared. It looks like the the creature is a very ultra rare spawn. If someboy found in the future a other location so please tell me and I will add this location to this solution.

Here is another location from the comments (THX Rier)
From Camp 1 in "Rotten Vale" jump down and run to the left down the hill, when you reach the bottom you'll want to start sneaking around the wall to your left with the Ghillie Mantle equipped. A mosswine will walk back and forth through the tunnel there and sometimes the Bristly Crake will sit on the mosswines back and ride along with it, be sure to leave your palico at the camp (I belive that the bird noticed me once because of my palico)
The location is really close to another capture a (rare) yxz creature achievement. So check both locations in one run to save a lot of time!