This is currently my pride and joy, the Ultraforge dragon. In my opinion one of the best dragon models out there. As you may notice it's far from finished at the moment, the paintjob needs some cleaning up, it'll get a rider for it, and it needs a base.
So how does this beast fit into the army? Well the idea is that it comes from a different planet (hence the massively different anatomy and colour scheme of the creature), possibly captured from Eldar Exodites or some similar beast-taming race, and used in battle by the most experienced dragon rider of the chapter. Its very existence is of course kept a secret as it would mean the downfall of the chapter if the Ordo Xenos got wind of it.
Here's some more shots to show you some details of the model: