Disc of Tzeentch
A Disc of Tzeentch is what happens when a Screamer gets field promotion and turns into a glorious steed for their Tzeentchen masters. The change is one of the most bizarre and abstract things to ever come out of the giant blue bird, since it turns a giant hell manta ray into a flying Beyblade. Apparently the change is made when Tzeentch reshapes the Screamer into a Disc using a series of potent rituals and sorcerous ceremonies and binds it with bands and chains of Warp-forged gold and silver.
The Great Shaper sometimes chooses to cover the Discs with eyes, to equip it with magical blades of living metal, or to add tentacles, feathers, scales, or bony plates. Through this reshaping process, the wild hunter becomes bound to its rider's will. The Champion or Daemonic Herald so mounted gains the Screamer's speed and ability to fly, and can take advantage of his Disc's psychically attuned senses, its lightning attacks, and the psychological edge of its unnerving scream. They also look cool/hilarious in it.
The Disc of Tzeentch has a much bigger and mechanical brother called the Aether Ray which is a Screamer/Disc on steroids. Capable of holding entire squads of Rubrics and a Chaos Sorcerer on its back and side while traveling through the dangers of the Warp. More akin to a Daemon Engine than a true daemonic mount.
In even rarer instances, a herald or champion is given a flying chariot pulled by two Screamers. In either case, the steed is capable of carrying its rider not only across real space, but into the warp as well. Such creations are usually seen at the head of vast hosts of daemons and have announced the doom of several planets in the Koronus Expanse.