I'm going to list every endemic life marked with a special border in the Hunter's Notes.

Terrestrial Life

Aquatic Life

Airborne Life

Treetop Life

Unclassified Life

In addition, some fish can spawn in a Great Size variant, which offers more research points and better items:

Great Fish

In addition to being larger, Great Size fish are also noticeably brighter than their regular counterparts.

When a Great Size fish spawns, it appears in addition to all the fish that are normally found at a fishing spot. If you see more than the usual number of fish, one of them is a Great fish.

Iceborne exclusive content

Eat for Felyne Zoomaster!! Combine it with the daily skill Felyne Biologist for even better chances.

Terrestrial Life

Aquatic Life

Sealord's Crestfish: a very long fish with glowing appendages, found during the night in area 8 of the Hoarfrost Reach. Unlocks the "Submerged Mystery" achievement.

Treetop Life

Blue Diva: a blue bird with a long tail, constantly singing. Found in the Ancient Forest (area 11, area 17, and area 17 again), the Wildspire Waste (three1 different spots1 in area 5), and the Coral Highlands (a total of four different spots in area 6). Can't be caught in some of these locations, as they are too far to reach with the Capture Net. However, instead of disappearing when startled, the Blue Diva will simply move to a different location in the same map. Keep chasing it! Unlocks the "Sweet Melody" achievement.

Unclassified Life

Catching any rare Moly unlocks the "Creatures of the Earth" achievement.

Great Fish

1: spots originally discovered by Steam user Destrukshor