Chaos Dragon

It’s whispered amongst the Northern tribes that somewhere in the icy peaks, there are Dragons, but not the ones of legend. These are the mutant spawn of Galrauch, the ancient Dragon who was corrupted by Tzeentch. The Chaos Dragons are thankfully rare and content to spend the eons buried in their lairs, brimming with hatred and plotting the destruction of all that lives. But when the Dark Gods call, not even these monsters can resist the urge to wage war for the Ruinous Powers. But they are fickle creatures, quick to turn on their allies should it give them the advantage.

—Reinholt Schent, Scholar of the Fantastic

A Chaos Dragon dimly resembles its uncorrupted kin. It has a massive bloated body that splits into two long trunk-like necks that end in heads of horns and fangs. One head smokes with the fires that burn in its gullet, whilst the other sends plumes of acidic gasses into the air to scorch the land. Thick scales cover the body in places, but in others, its organs and muscles are revealed, pumping the corruption through a system of exposed veins and arteries. Its fiery heart beats within, sending showers of sparks onto the ground as it passes.

Source

 * Warhammer Fantasy RPG 2nd ED -- Tome of Corruption (pg. 114).