you get a chaos dragon (or really any dragon with a bit of converting) and use the torso from some tzeentch (or any other chaos character) with some dragon riding legs....
in other terms, just pick up a chaos dragon and paint the rider in a tzeentch-scheme (hehe funny pun), or if you want, cut the rider in half and keep the legs, while cutting a tzeentch champion and glue him to the other legs.
That is like asking where is the Warriors with mark of Tzeentch! they aren't! It's not that diffucult, and it would be idiotic for GW to make a dragon for each of the gods! like with the warriors it might just come down to painting.
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