Defiler
The Chaos Space Marines don't have a lot of their own vehicles at their disposal; whilst the loyalists have all kinds of nifty-cool tools fresh out of the Adeptus Mechanicus' foundries, the Chaos Marines have to make do with equipment that is antiquated in contrast. In many cases, their technology is capable of bridging the gap; the Chaos Marines field a number of weapons no longer in use with the Astartes (man-portable autocannons, for example). In other cases, their technology is inferior; the Chaos legions lack anything resembling Landspeeders, Whirlwinds, or skimmers, for example.
But it's not all bad. Chaos Space Marines are nothing if not inventive. And thus they came up with a vehicle that is as horrifying to look at as to be attacked by - the Defiler.
Often jokingly called a giant enemy crab, the Defiler was supposedly first-concieved by Failbaddon, but its true origins are tacitly known to lie with the Word Bearers. Attempting to deal with the fact that Chaos had inferior Dreadnoughts to the loyalist chapters, the Defiler has no actual crew - instead, it is a heavy mechanical creature fueled and controlled by a bound Daemon, driven borderline-mad by its captivity. Imbued in the rune-inscribed hull of the machine, it proceeds forwards at the whim of the Chaos Space Marines unit it serves, venting its unholy fury on any target its masters deem hostile. It is not a subtle weapon by any means; it is huge, heavy, well-armored, and bristling with weapons and spiked blades, as deadly in close-combat as it is from afar.