Pteryx are bird like creatures that you'll encounter throughout the DLC. There's at least 21 of these enemies during the main quest alone. There are a bunch more of them in the sewers and undersewers. They'll respawn at the Eryie Peak location as well.
People have reported this achievement not popping for them. It's thought that this might be like the "Exterminator" Scav achievement from the Dead Kel DLC, where if you one-shot the enemy, it doesn't register the kill. So make sure you don't use your most powerful abilities/spells on these enemies.