Muggles' Guide to Harry Potter/Magic/Lethifold

Overview
The Lethifold, a large, flat creature somewhat resembling a dark cloak or blanket, is one of the most dangerous (and mysterious) magical beasts in the wizarding world. It hunts at night, giving it an advantage over the (sleeping) creatures which it preys upon.

Extended Description
In appearance, a Lethifold is quite similar to a motile cloak, which travels by rippling itself across the surfaces it is lying on. It apparently has limited ability to fly by flapping itself through the air. Its chosen attack technique is to surround and smother its prey. The only spell known to stop a Lethifold is the Patronus charm; this was discovered by Flavius Belby, possibly some relation to Marcus Belby. As quoted in Fantastic Beasts and Where to Find Them by "Newt Scamander", Belby writes:


 * Near one o'clock in the morning, as I began at last to feel drowsy, I heard a soft rustling close by. Believing it to be nothing more than the leaves of the tree outside, I turned over in bed, with my back to the window, and caught sight of what appeared to be a shapeless black shadow sliding under my bedroom door. I lay motionless, trying sleepily to divine what was causing such a shadow in a room lit only by moonlight. Undoubtedly my stillness led the lethifold to believe that its potential victim was sleeping. To my horror, the shadow began to creep up the bed, and I felt its slight weight upon me. It resembled nothing so much as black cape, the edges fluttering slightly as it slithered up the bed towards me. Paralyzed with fear, I felt its clammy touch upon my chin before I sat up bolt upright.
 * The thing attempted to smother me, sliding inexorably up my face, over my mouth and nostrils, but still I struggled, feeling it wrapping its coldness about me all the while.
 * Unable to cry out for assistance, I groped for my wand.
 * Now dizzy as the thing sealed about my face, incapable of drawing breath, I concentrated with all my might upon the Stupefying Charm and then - as that failed to subdue the creature, though blasting a hole in my bedroom door - upon the Impediment Hex, with likewise availed me naught. Still Struggling madly, I rolled sideways and fell heavily to the floor entirely wrapped in the lethifold.
 * I knew I was about to lose consciousness completely as I suffocated. Desperately, I mustered up my last reserve of energy. Pointing my wand away from myself into the deadly folds of the creature, summoning the memory of the day I had been voted president of the local gobstones club, I performed the Patronus Charm.
 * Almost at once I felt fresh air upon my face.
 * I looked up to see that deathly shadow being borne into the air upon the horns of my Patronus. It flew across the room and swiftly out of sight.

Analysis
The name of this creature appears to be based on "lethe" (sleep) and the idea of enfolding; thus something which enfolds the sleeper.

If should be noted that in Harry Potter and the Order of the Phoenix, it was noted that an old robe had tried to strangle Ron while he was helping clean up Headquarters. There has been speculation on several fan sites that the "cloak" may have in fact been a Lethifold. There is nothing in the book that would confirm this belief, the attacking object is never described as anything except a cloak.

The fact that the creature is vulnerable to a Patronus charm, perhaps along with its technique of suffocating its victims, leads us to believe there might be some relationship to the Dementor; perhaps, in time long past, Dementors were created out of Lethifolds.