Stalk Tank
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.
Why the vehicle is called a Tank when it's a walker without a turret and doesn't fulfill any of the battlefield roles of any kind of tank due to its primary target being infantry is anyone's guess. Probably just an informal designation made by people who don't know military vehicle designations like your average uneducated dumb motherfucker who joins up with the Lost and the Damned that just ended up sticking because to be fair, it's a really badass name.
It can be thought of as essentially a Chaos version of the Imperial Sentinel walker, with roughly comparable armament and armour. It can be armed a bit more heavily than its Imperial counterpart, but is also a bit more fragile and isn't as capable of unconventional forms of deployment as the Sentinel and isn't quite as modular as only a single Stalk Tank variant is known. It still fulfills a broadly similar role as a very light, highly mobile walker that can provide decent firepower that would otherwise need to be lugged by heavy weapons teams at an affordable cost while still being too much for infantry without some anti-vehicle firepower to deal with. As the old maxim goes "when all you have is an assault rifle, the difference between an M4 Sherman and M1A2 Abrams tank is mostly academic."
The Unit Itself in TT[edit]
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 AKA A Combi-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.
In the new CSM codex, GW is trying to tell us that the Maulerfiend is a Stalk Tank. Riiiight.