If you're a trophy/achievement hunter, you may have noticed some hidden trophies on your list, which, if you peek, charges you with capturing rare endemic life... and one fish. Some of these appear as delivery requests, while others can be done free-form, but they'll all be covered here. Who doesn't love trophies, after all? Plus, it does involve a bit of running about on expeditions, which is exactly what you need to be doing to further your ongoing Elder Dragon investigations. Two birds, right? Speaking of two birds...
Research Help: Phantom Bird
Look out for the Endemic Life Researcher in the Ancient Forest, and when you find her there, travel to the Ancient Forest. You'll find her in her usual spot along the western end of AREA 5, near the Northeast Camp. Talk to her and she'll tell you that the fruits of your labors - and her research - has led to her finding a new critter for you to hunt. To this end, she'll give you the delivery request Research Help: Phantom Bird, charging you with finding this aforementioned critter, which she provides little information about.
That's what guides are for though, no? This "Phantom Bird" she's after just so happens to be a Downy Crake, a fluffy bird whose location isn't best answered with a "where", but with an "on what". The Downy Crake isn't found on its own, as it's a symbiote that travels on the backs of other, larger monsters. In this case, you can find them hitching rides on Apceros in the Wildspire Waste. Your best bet for finding them is to simply travel to the Southwest Camp in the Wildspire Waste and check the backs of the three Apceros that spawn there. They're gray little puff balls of birdy cuteness that line up on the backs of their Apceros host.
Unfortunately, there's a further complication, as for critters that roost on the backs of loud, lurking armor dinosaurs that often end up as prey for other, larger dinosaurs, they're rather flighty. Sneaking may not be sufficient cover - to bag these birdies, you're going to want to make use of your Ghillie Mantle. If that's not annoying enough, the Downy Crake won't show up every time you enter the area, so you'll likely need to travel between areas (traveling between camps on the same map is not sufficient).
So... equip a Ghillie Mantle, travel to the Southwest Camp in the Wildspire Waste, head out into the AREA 1, use your Ghillie Mantle, then search the backs of the Apceros there for your birds. If you see them, get close and catch them with capture net. When you return to the Endemic Life Researcher she'll give you a Gold Wyverian Print.
Snuggles for All
Capture a fluffy, snuggly creature.
Bagging a Bristly Crake
If you've still got a mind to go after more birds, there's another rare critter out there. There's no quest and fanfare this time, but there are many similarities. This bird - the Bristly Crake - rides on the back of another symbiotic animal, this time a Mosswine, and only those in the Rotten Vale. It's also very flighty, so you'll again need to make use of your Ghillie Mantle.
In the Rotten Vale there are two locations you can find a Mosswine, and hence, the Bristle Crake. From the Southeast Camp, jump off the cliff and turn west to reach the wider platform in AREA 3. After reaching this larger area, immediately turn south to find a hole you can crawl through, beyond which (in AREA 5) you can find a Mosswine.
From the Central Camp, leap off two cliffs to the northwest and northeast, then continue north and turn west and climb up a ledge to reach the chamber (AREA 10) adjacent to the Odogaron's lair (AREA 13). Head to the southern end of this area to find a hole you can crawl through to reach the Grimalkyne cave (AREA 14), then turn west to reach the chamber the Grimalkyne actually occupy. Once inside, turn south and climb up a ledge to reach the walkway overlooking the fishing pond, on which a Mosswine lurks. Travel between areas, as the Bristly Crake may not appear in the Rotten Vale every time, check both Mosswine locations, and repeat as necessary.
Bristles for All
Capture a stiff, bristly creature.
Poaching a Prism Hercudrome
This next creature can be easier or more difficult to catch, depending on your luck. You don't need a Ghillie Mantle, and you don't need to look for another creature, this target's location is static. Unfortunately, your target's schedule is erratic, and it'll only appear during the dawn and the dusk.
First, the location. Travel to the Northeast Camp in the Coral Highlands, crawl out through the hole to the west, turn north and drop off a cliff to the north, then drop off another cliff to the west, and finally jump off a third cliff to the south. On the lowest part of AREA 10, turn west and crawl through a hole, then follow the linear path ahead until you reach a waterfall-fed pond. At this water, turn north and follow the stream northwest until it converges with another stream before finally running off a cliff. Where the stream you followed unites with another, turn northeast and head up a ramp and follow the path as it winds around some large mushroom-like plants. On the northern end of larger mushroom you may find your target - a Prism Hercudrome. If your insect isn't there, you'll have to return after some time has passed - traveling between maps should do the trick.
Capture a creature that glitters like a rainbow.
Research Help: Capture the Ancient
If you've completed all the other delivery requests for the Piscine Researcher and you see him again in the Rotten Vale, go pay him a visit. You'll find him along the eastern end of AREA 9, and when you talk to him he'll ask you to catch one final fish, starting the delivery request Research Help: Capture the Ancient.
You won't find this creature in the Rotten Vale, but amusingly enough, you can find one where you may have caught the last fish (the Platinumfish). Travel to the Eastern Camp (AREA 8) in the Elder's Recess and check out the fish-filled pool to the east, where, along the northern end of the pool, you may find the fish you're looking for. You'll know it if it's there, as it's rather large and has bright blue spots on it, although it can be somewhat far out to the north, possibly obscured by crystals. Catch this prehistoric fish, this Petricanths, and talk to the Piscine Researcher afterwards to obtain another Gold Wyverian Print.
A Living Fossil
Capture a fish known as "the living fossil."