Ahsa

Sea monsters are giant aquatic sea creatures that resemble serpents and live in the methane oceans of Titan. It is unknown if these creatures were always present on Titan, or if they arose or were introduced to the planet following the onset of the Golden Age. One creature can be seen through the windows surrounding the arcology's CPU core in the Festering Halls during the mission Utopia. Ghost mentions that the arcology's sensors can detect a large object moving under The Rig, and although the sensors can't determine what it is, this is likely the same creature.

Trivia

 * There is an ongoing debate about this sighting. Some claims that the players mistook parts of the Arcology for a Sea monster, much like how in Destiny players thought Batadactyls on Venus were Ahamkara. However, the images about the three "wheels" were taken below the ocean level, an area where the player is not normally supposed to access. This means that the wheels could be a smart solution the 3D artists used to render the monster scaled back while moving from left to right and from above to below the water level, giving a good impression of a sea serpent, supporting the idea of a real living gigantic creature under Titan ocean.
 * Below the Waves in the mission Utopia and Savathûn's Song (strike), a huge worm-like being can be seen drifting beneath the waves. It's size varies from less then a tenth of the windows, to over half the size of the windows. It's general shape and tail appendages may tell us that this is a monster Worm. Perhaps a Worm God. But until we can get a detailed look upon this being, if ever, it is unknown.

List of appearances

 * Destiny 2