The giant dianoga was a potential subspecies of the common dianoga. Growing many timers larger than an average dianoga (being able to dwarf a human), giant dianogas sported several barbed tentacles and a gaping maw which was not unlike that of a sarlacc creature.
Only one known specimen was recorded in galactic history; it inhabited the chamber of the sewers directly underneath the lobby of Prince Xizor's palace on Imperial Center. In 4 ABY, when a Rebel taskforce led by Luke Skywalker attempted to infiltrate the palace via the sewer system, the dianoga was killed by Dash Rendar in 4 ABY, who was forced to kill the creature by destroying its eyestalk after being trapped in its chamber.
Behind the scenes
The giant dianoga made its first and only appearance in the video game adaptation of Shadows of the Empire as the boss of the "Sewers of Imperial City" level. Since the game's storyline directly contradicted that of its source material, Dash's encounter with the beast may or may not be considered canon. However, that doesn't necessarily discount the canonical status of the species itself.