The Cockatrice is a Moa-like hostile mob that spawns in darkness in the Aether, similarly to Overworld hostiles. It has darker coloration than Moas and shoots Posioned Darts at you. They are highly mobile and move quickly. Taking them out with a ranged weapon is recommended.
Previously, the spawn rate of cockatrices was quite high; now, fortunately, their spawn rate has been significantly lowered and seeing one is rare.