The Onager Dunecrawler is an assault walker used by Adeptus Mechanicus Skitarii. Based on technology dated from the Dark Age of Technology, the Onager Dunecrawler is based on the Mars Universal Land Engine (or M.U.L.E.), which in turn was fashioned by famed technoarchaeologist Arkhan Land (Of the Land Raider fame) as a form of transport but was soon modified into a weapon of war. No word on why it's still not used as a transport like a Rhino, a dedicated APC is something the Adeptus Mechanicus seems to lack.
Despite looking like the demented offspring of a Sentinel and a giant crab, the Onager Dunecrawler is significantly more advanced, better armored and better armed. Able to carry a collection of advanced weaponry not seen since the Great Crusade such as a Eradication Beamer, Phosphor Blasters, Neutron Laser, and a coaxial Cognis Heavy Stubber, one must wonder how easy wars would have been fought if the Mechanicus actually decide to export such weapons to the Imperial Guard but alas, it is all wishful thinking. It can also be equipped with an Icarus Array, a powerful anti-air package comprising twin Autocannons, a Gatling Flak Launcher, and Missile Launchers making it a bane to most enemy flyers. Recent technological developments, probably from the big cog himself, also now allow for the Icarus Array to efficiently target any vehicle that dares to propel itself forward without using sanctimonious wheels, treads, or mechanical appendages, much to the ire of those that employ filthy xenos technology.
The Dunecrawler maintains a crew of two fused with their Onager host in a symbiotic relationship similar to that of the Princeps and Moderatii of the Titan Legions. Its gunner is a former Skitarii Ranger that fires the weaponry while the driver is a Skitarii Vanguard, whose augmented body allows him to be resistant to the harmful energies being spewed out; allowing him to immerse himself in an electro-amniotic fluid that allows direct communication with the walker's Machine Spirit....it's similar to the Matrix films really. Treated like expendable living batteries, the crews inevitably die over time and are promptly replaced.