Chaos is nothing if not inventive.
When the Defiler proved effective in combat situations, the Traitors of the Lost and the Damned took note - here, they argued, was a useful and powerful weapon, one that the Traitors could make use of - and fully-intended to do so. However, they realized quickly that the design could be modified into a useful fast-attack and reconnaissance vehicle. The end result was the Stalk Tank. Unlike a Defiler, the Daemon-Engine of the Stalk Tank is intended to work alongside a living driver; the result is that driver and Tank alike work together in causing carnage and mayhem, though this means that the Stalk Tank is a bit more vulnerable to having its driver knocked about than the Defiler as a result. The design worked, and after first being popularized by the Khornate forces of the Blood Pact, it was quickly co-opted by allied forces before being gradually spread to every group of heretical scum in the Segmentum.
The Unit Itself in TT
Simply put, an awesome bit of somewhat-fragile Dakka for a nice, low points-cost and the ability to be fielded in squads. It's got the same armor as a Spess Mehreen METAL BOX, and lacks the Scout ability of a Sentinel - but it makes up for this by being substantially better-armed; by default, it has twin-linked Multilasers, and it can upgrade to use a Lascannon, twin-linked Autocannon, Twin-Linked Missile Launcher, or twin-linked Heavy Flamer. Unlike the Sentinel, the Stalk Tank can also mount pintle-mount weapons; a Havoc Launcher, Heavy Stubber, or a Twin-Linked Bolter. It also boasts the Fleet rule. Sadly, for all their firepower and speed, they're lightly armored, and will evaporate outright if subjected to sustained anti-tank weapon fire.
Being a Lost and the Damned Vehicle, there's no official model for it; this is because you're expected to build one. Some players mod Defilers into them; others scratch-build them from bits and pieces of Rhinos and Chimeras. Others go full-on Ork and build them from decidedly-not-40K stuff, with just enough bitz scrabbled together to ensure it can pass inspection. Some players build them to resemble mantises or spiders; others just make them look like tanks or APCs with feet or quadrapedal Sentinels.