Only one specimen was recorded in galactic history. It inhabited the chamber of the sewers directly under the lobby of Black Sun leader Prince Xizor's palace on Imperial Center. In 4 ABY, when a Rebel Alliance team led by the renowned Rebel pilot and Jedi in training Luke Skywalker attempted to infiltrate the palace via the sewer system, the dianoga was killed by smuggler Dash Rendar after he was trapped in its chamber. Rendar bested the creature by firing at its prominent eyestalk.
The giant dianoga made its only appearance in the video game adaptation of Star Wars: 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 within the Legends continuity. However, that doesn't necessarily discount the canonical status of the species itself.