It's notoriously hard to obtain due to many enemies having variants or only showing up in one place.
Some common enemies missed are:
Blurst - Their swarm is vulnerable, let them explode/hatch. They inhabit in Indi.
Donaught - The drones spawned during the last boss battle.
Trap Claw - These come in 3 colors, Default (Pink), Meta (Red), and Gamma (Blue). Search the upper-left of Eribu, the rare one leaves a stream of light.
Loop Diatom - These enemies come in two colors; Mulberry (Default) and Violet.
Volg - Found in central Edin, a hanging green pod in a 2x5 room.
Mutant - Comes in two colors.
Swarmily - Found in Eribu.