Kubu is an ancient Mesopotamian demon. He was known as the "little one" who never "saw the sun," "suckled his mother's milk," or "knew his own name," implying that he is a projection into life of a stillborn, miscarried child. Because he can never be recognized and accepted by the living world, he is forever trapped between life and death, never growing older. Desperately envious of living children, Kubu retaliates by killing them with ills and diseases, trapping them with him forever in limbo.
Although Kubu is a frightening creature, and doubtless was used to scare children into behaving, his mere existence was reassuring to the ancient Mesopotamians. As 'living' proof that their stillborn children were not really lost forever, just off in the darkness somewhere playing with Kubu, he consoled stricken parents and gave them a way to remember a child that would never have a name of its own.
Kubu has come down through the ages in various other nursery tales, fairys, and bogeymen, but perhaps his most well-known successor is Peter Pan. Peter also never grows up - in fact, he flew away from his mother to avoid growing up, and when he tries to return, he finds that he's been replaced and can't go back. He chooses to glory in his "Betwixt and Between" state, and starts collecting children who fall out of their carriages - the Lost Boys - and steals others straight out of thier nuseries - the Darling chlidren. Forms of stillbirth? Perhaps. Perhaps also, seen in this light, both Kubu and Peter are expressions of the fear that the lives of children are not controllable by their parents.
Interestingly, 3000-year-old Mesopotamian instructions for glassmaking require that images of Kubu be placed on the furnace. Are Kubu and children also somehow related to glass-making and fire? Maybe (this is an intuitive leap of my own) the Kubu images by the fire would keep children from falling into the fire, or from cutting themselves on glass shards, by showing Kubu that he was, indeed, remembered by the living.
My sources are Diane Purkiss' book At the Bottom of the Garden: A Dark History of Fairies, Hobgoblins, and Other Troublesome Things and the website of the Corning Museum of Glass, http://www.cmog.org. I found no other references to Kubu the demon anywhere else. If anyone's familiar with him, send me a /msg.