From Halopedia, the Halo wiki
Halo: Combat Evolved
The Forerunner Tank, as intended for the first Halo game, was concepted by Marcus Lehto as a heavy vehicle intended for use by the Forerunners. Whether the vehicle progressed beyond the concept phase or reached any active development is unknown.
The level named Forerunner Tank was only revealed on the Halo 2 cutscene commentary contained in the Halo 3 Legendary Edition's Halo 2 cinematics commentary. The purpose of the level was to tie in plot between the end of the level Regret and the cutscene after the end of the level Quarantine Zone. The level was said to be placed after the cutscene in Quarantine Zone where the Arbiter is pushed down the index chamber by Tartarus and before the cutscene where the Arbiter meets the Master Chief and talks to the Gravemind. This level was supposed to be the Gravemind's introduction; in the level, the Gravemind's tentacles were going to slam down all around the player, and move behind doors like racing freight trains.
The level would have described the Master Chief's adventure through the bowels of Delta Halo before his eventual capture by the Gravemind; the "Forerunner Tank" providing the namesake of the level was only described as something "... awesome, that blows things up, glows a lot from little windows and moves real fast." When asked why he wanted the Forerunner Tank in the game, Jason Jones' response was, "Awesomeness would ensue". The level's replacement was the short cutscene at the end of Regret; the level's removal resulted in the long series of different cutscenes between Quarantine Zone and Gravemind.
In the Halo 2 level Sacred Icon, the gondola is a heavily modified piece of moving geometry originally intended for Forerunner Tank.