Two rare pets can be captured during the final Piscine Resarcher and Endemic Resarcher quest and you will be awarded with an achievement for doing so. The first one is Petricanths, an ancient fish, which can be captured in the Rotten Vale or the Elder's Recess. The easier spot is in area 15 of the Rotten Vale. Start from the Central Camp and descend down to the shown spot in area 15 near the Grimalkyne camp. Look out for a blue glowing fish. This is the Petricanths. It is now always there, so if you can only spot three golden fish you have restart and look again. Catching the Petricanths will unlock the 'A Living Fossil' achievement.
The second one is Downy Crake, a rare bird, which can be captured in the Ancient Forest, Wildspire Waste or Coral Highlands. Go to the Central Camp of the Wildspire Waste, but take your Ghillie Mantle with you, as it is necessary to capture the bird. It will appear on the back of Aptonoth or Apceros. So look out for them in area 8 and turn yourself (and the camera) away from them while wearing your Ghillie Mantle. As soon as you hear some birds chirping turn around and capture it. This will award you the 'Snuggles for All' achievement.
The Bristles for All achievement can be unlocked at daytime by capturing the Bristly Crake. Go to the Central Camp in area 11 of the Rotten Vale. Take your Ghillie Mantle with you and leave your Palico behind as it would scare the creature. Head for area 15 as you did for the Petricanths, but before reaching the pond climb up the ledge to your right. Here is a Mosswine. The Bristly Crake has a rare chance of spawning on the back of the Mosswine. If it spawns, capture it. If not, just restart and try again. As this is spawn is rare, this process may take a while. Just don't give up!
The Rainbow Bright achievement can be acquired in the Ancient Forest during dusk or dawn. Start at the Southwest Camp in area 1 and head to the southeastern point in area 1. Coming from the north look out for the Prism Herkudrome sitting to your right on the wall.
