The Newborn is a new breed of Xenomorph, which perfectly combines the DNA of a Human and a Xenomorph into a highly efficient killing machine.
It was created when scientists of the United Systems Military cloned the body of Ellen Ripley from DNA apparently removed from her upon her crashlanding on Fiorina 'Fury' 161, which contained a developing Xenomorph Queen (although how her own DNA could contain that for the Queen is unknown). Once removed, the Queen continued her cycle of laying eggs, but developed a second stage, which involved the creation of a womb - inherited from the DNA exchange from Ripley.
This, the Newborn, was the offspring that emerged from the womb. It had softer skin, rather than the hard, rigid body of other breeds. It also had more human proportions, as well as a skull-like face, with eyes deep in the sockets and feral fangs. It identified Ripley 8 as its mother and killed the Queen by smashing the front of her face, not accepting a Xenomorph as its mother. The Newborn also lacks the offensive adaptations of the purer Xenomorphs: it has no inner jaw or tail. However, its teeth are still sharp enough to penetrate bone and it has incredible strength, being stronger than a queen. It also lacks dorsal tubes and has red blood, although the acidity of this blood is unknown. Unlike other Xenomorphs, it displays some emotion which is uncharacteristic of Xenomorphs, but expected of a human. It acts like a human baby, except with more violent instincts and too high a strength to carefully control what it is doing. This is shown when it was reportedly playing with its kills, humming tunelessly, like a human baby who doesn't yet understand the repercussions of its actions.
It died when Ripley purposefully damaged part of the escape ship's window with her acidic blood (since she is partly alien), causing the Newborn to be sucked into shreds through the small opening.