Mutalith Vortex Beast
Basically the Chaos version of the Haruspex, if it weren't for this abomination being around before the Haruspex was made. The Mutalith Vortex Beast is a creature that can be found in Warhammer Fantasy/Age of Sigmar and more recently has made its debut in Warhammer 40,000.
Honestly, its aesthetically one of the ugliest looking from the Chaos line, beating out even They-Which-Must-Not-Be-Named, and while the model quality is great, a great rendering of a hideous bag of flesh and slimy tentacles is still a hideous bag of flesh and slimy tentacles. Not even Nurgle can help its ass in the makeover department.
Mutalith Vortex Beasts are terrifying abominations of nature, which have been mutated beyond all reason by the power of Chaos and is now a conduit of raw abstract and chaotic powers of absolute Bullshit. Because of this, any servant of the Dark Gods who wishes to use these avatars of living tentacle rape, risks suffering from mutation or madness (atleast more-so than usual).
The Mutalith Vortex Beast differs from the average disgusting thing in a striking manner: a vortex of raw Chaos echoing the shape of the 8-pointed star looms up from its back, with a shard of the great eye that crowns Magnus the Red's tower in the star's center. Nine huge tentacles extend out of its mouth, each of which is dripping with ichor. Finally, it possesses two tails that end in mouths, tongues and claws. It comes with an alternate head that has no tongue; instead there is a grimacing maw and growths protruding from its face.
Mutaliths also aren't born (un)naturally in contrast to most Chaos Beast or God-aligned creatures. Instead, they are formed by the collapsed fusion dance of multiple They-Who-Shall-Not-Be-Named. When under the exposure of the warp energies that flow around the surface of the Planet of the Sorcerers, they undergo another form of mutation. In most cases, the would-be mutalith is unable to handle the mutations it goes through, and collapses on itself, turning into ooze. But when Tzeentch feels like it, the shall-not-be-named entity will instead continue to mutate into a full Mutalith with continued warp-energy exposure. Additionally, through a stroke of good/bad luck, Tzeentch can also turn one of his chosen champions into a Mutalith, instead of achieving daemonhood.
Basically, they're the daemon prince version of They-Who-Shall-Not-Be-Named. Either becoming more powerful with a god's patronage, or devolving further into nothingness.
So far, only the Thousand Sons seem to be able to both tame and master control of these beasts.