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Aside from it's eyes and coloration, the Hatchling does not appear much different from a Xenomorph. The difference lies in it's intelligence, as it -- like King -- can talk and has the capacity to use weapons instead of just it's body.
The Hatchling was born shortly after King was and thinking the other the better "version" of the White hybrid, the two battle it out. However, the Queen intervenes and kills the Hybrid. The ship the fight takes place on is detonated by Big Mama, Caryn Delacroix, and her allies as well as the Hatchling are able to escape.
It now accompanies her and it's presence is accepted even by the Yautja.