You've seen them. We've all seen them. Men; fathers; young fathers, especially. They walk the streets with their offspring on their shoulders, toddlers with fat little legs wrapped round their dads' necks, a tower of generations that somehow never topples. In prehistoric times, a man would be satisfied with the task of re-purposing mammoths for food. The modern male needs reassurance that he can make his fatherly duties look macho and look twenty-first-century at the same time, and will therefore let his child (usually a son) ride bareback on his head. These men stalk the cities, occasionally pointing at things that look interesting but not too feminine, shouting "what an enormous bridge!" while secretly thinking see, I'm lifting my own child's weight, that proves I'm sensitive yet strong.
No, I know what you're thinking. You're thinking that I'm going to present these shoulder-childer as parasitic aliens, controlling the minds of the ones they straddle. Not at all.
Step back. So far back that you can see both Earth and Raxacoricofallapatorius in the same shot. It should be obvious, from the Godfather-bloodlines we've already seen, that the people of the latter planet are much too proud of their genes. Their criminals have a profound sense of family loyalty, even if they are what any galactic Sweeney might call "slags". If you know anything about Raxacorican breeding procedures, then you'll understand why, but for now it's enough to say that appearances are important. A phrase like "blue blood" would be taken literally among these breeds. They feed on bio-stock more often than food (note their appetite when faced with the real stuff, plus their aversion to certain condiments), and the bio-stock is tailored to genetic groups in such a way that they really do end up with heraldic cardiovascular systems. Although this is most obvious when they explode.
Of the Raxacorican families, the Kilquireen are more blue-blooded than most. They're known for their turqoise skins, as well as their visible and strongly-pulsating veins, which some have described as giving them an appearance of great... ahem... intensity. Brainy on the outside. This is a deliberate ploy on their part, but then, what isn't? Humourless, pragmatic, and trained in a form of logic that denies the possibility of non-Kilquireen achievement, this is a bloodline that calls itself aristocratic because it believes it deserves to be.
They go out of their way to display their self-involvement. Their crania are massively over-extended. Their skulls stretch backwards from the face, rather than being rounded-off in the fashion of (say) the Slitheen: imagine the head as the latter part of a wasp, a bulbous, hard-shelled sac. A bishop's hat of a noggin. Because of all the extra brain-capacity...? No, of course not! Because they've grown their heads that way, in an adolescent coming-of-age rite that ties wires of futurite around the still-growing skull. Supposedly, this allows them to look both ways before crossing the timelines. It also squeezes their head-flesh into a shape which will, at a glance, cow any lesser species into believing they're the master chess-players of the galaxy. They planned it that way, however, so maybe they are master chess-players.
But this brings a problem, when moving among those people whom we'd call humanoid. A Slitheen can compact its girth into a skin-suit with mere dimensional corsetry, but a Kilquireen...? Its head can't be squashed so easily. And sometimes, one of their number will descend to our level, perhaps to cauterise the infection brought by one of its sibling-houses. Complete "re-rendering" of a world may occasionally be in order, and some other disguise may be needed in the midst of these cultures. Hats might be considered a good move, yet the higher the status of the Kilquireen, the more exuberant the head. A turban can only conceal so much.
Close-up on that man, with his child on his shoulders. You knew, didn't you? Something about his sense of superiority, the belief that the infant lump on his back put him on a different level of morality. Now he reaches up, above his own head, to the scalp of the child. He begins to unpeel the skin, using the zip that's hidden beneath the boy's pudding-bowl haircut. The space beyond reveals bright light and cyanide-coloured flesh. The zip goes down over the forehead, down through the chest and clothing, down to the belly-button... and there, the torso meets the man's own skin. Now the zip-line of light continues down his forehead, down his chin, splitting his throat in two. The entire child, previously so alert, becomes a floppy hood behind his neck.
And there he stands, the Kilquireen elder. His body, a skeleton wrapped in aquamarine blubber. His head, so immense that it could only be concealed by the outer skin of a three-year-old and kept afloat by its own telekinetic smugness. His eyes, big and black and powerful, seem to say: "This is most disagreeable. I've got better things to do."
Ah, but here's the problem.
They disguise their heads as children. But to make the disguise perfect, the children have to speak. Not difficult, for something as psychically well-drilled as a Kilquireen. The upper mouth of the skin-suit is designed to flap about a bit, so the wearer simply has to project an infantile voice from the upper part of his or her skull. Hah! Child's play. In fact, the Kilquireen rather enjoy it. And given that the very matter of their brains is trained to obey from an early age, sometimes you can even find a pair of lips manifesting itself in the forehead. An extension of whatever they have instead of a pineal gland.
Oh, yes, they enjoy it. A little too much.
And one day, a Kilquireen - revealing himself to his prey on Earth - explains that due to contamination by Slitheen technology, his victim will "have to be removed on a permanent basis".
And the lips on his forehead blow a raspberry.
And the Kilquireen says: "Behave yourself."
And the lips say: "G'wan, stiff him."
And the Kilquireen suddenly realises that... he's not controlling the lips any more.
The Kilquireen of Raxacoricofallapatorius. Serious. Amoral. Repressed. And now discovering that by pretending to be men with children on their backs, they've put children on their backs. Children who know everything they know, and have an awful lot of pent-up aggression to get rid of.
Let's be honest, Earth has turned them schizophrenic.
At least the Slitheen were predictable. From now on, the Kilquireen will blow up planets just because they didn't get any jelly.