Unreleased Vehicles

Agrimotor

This was obviously never intended for release, but seemingly existed as an internal test vehicle/inside joke. Fun fact — according to FTR, something similar to this did exist in pre-release versions of the game as the “Traktor”, which could shoot from its exhaust (attached to the vehicle’s side facing forwards, rather than upwards). However, this obviously never made it into the release version.

The Neubaufahrzeug was a pre-war German heavy tank fitted with three turrets and two main guns (37 mm + 75 mm, both of which were attached to one another in the main turret).
In-game, the vehicle was leaked more-than-once: first in the 0.8.0 icons leak, and later in the 8.9 ASAP video, which showed the vehicle as a tier 3 premium in the German tree (see below). The vehicle even had a completed model; however, it was never implemented due to the multi-gun mechanism not existing at the time (and still not existing for guns of different calibres).

According to FTR, this vehicle was originally intended to replace the IV Jagdpanzer 38(t) “Hetzer” in the tech tree. Supposedly, it would’ve come with an adjustable suspension which would allow it to lower its silhouette in exchange for not being able to move at all (almost like an extremely-early version of siege mode!). Notably, the Jadgpanzer 38(t) was not historically named the “Hetzer” — the name instead came from this vehicle (the E-10).

Brummbär and X Sturmtiger

Brummbär was an assault gun based on the Panzer IV chassis, fielding a 15 cm StuH 43 L/12 cannon in a fixed casemate.
Sturmtiger was an assault gun based on the Tiger chassis, fielding a 38 cm rocket mortar in a fixed casemate. (Fun fact: the shells for this vehicle were so large and heavy that it came equipped with its own crane on the back, purely to assist with loading ammunition through the rear hatch.)

To start off, this vehicle was initially considered for introduction as a tier 10, similar to the FV215b (183). However, the vehicle was quickly canned, and rumors of the vehicle one-shotting multiple tier X heavy tanks in an internal round of testing (including an X Maus allegedly being destroyed purely by splash damage) should probably explain why.
From there, the model was hidden in a garage on the now-removed Severogorsk map as an easter egg (until the map was removed in Update 0.9.5 [December 2014]): https://youtu.be/K23EGf9a1QE

However, that wasn’t the end — the vehicle made a surprise return as part of the 10th Anniversary in 2020, where it became a playable vehicle in the “Crouching Sturmtiger” fun mode. In this mode, the vehicle featured damage numbers befitting of its calibre: 3,000/7,000(!), though the vehicle itself was also given 10,000 health and extremely-buffed mobility, and could only meet other Sturmtigers in the mode (no one-shotting of any Maus with splash damage here).

Not only that: while the PC version does not currently feature either vehicle (and will likely never feature the Sturmtiger itself), both vehicles do actually exist on WoT Console, as part of a tech tree branch consisting purely of assault guns with derp guns. The branch was added in October 2021, and consists of the following vehicles:

V StuIG 33 B, a historical vehicle fitting a 15 cm s.I.G. 33 L/11 cannon on a StuG III chassis
VI Brummbär
VII Sturmpanzer, a vehicle similar to the VIII Jagdpanther II fitted with a 21 cm(!) Morser L/14.5 cannon
VIII Sturmtiger (P), fitting a 22 cm Mrs 531 (f) cannon into the VIII Ferdinand
IX Bär, a superheavy Krupp mortar project mounting a 30.5 cm Morser L/16(!!) on an elongated Tiger chassis
X Sturmtiger

Other vehicles:

StuG III H

Not much is known about this one — the icon is the same as the one used for the VI Jagdpanzer IV, which had already existed at the time. However, this icon was very clearly labelled “StuGIIIH”. Perhaps this was done in error, either by Wargaming or one of the leakers?