Sea creature, sea creature (Thin Ice), or the like -- see Talk:Creature (Thin Ice)#Sea creature.
Talk about it here.
The creature was massive in size, easily several times that of a whale. It shared many similarities with lure fish, namely the overall shape of its body, its fins and its bioluminescent lure on its forehead. The creature was female and could produce several offspring who were the size of regular lure fish.
The creature's race were extremely long-lived and could potentially live for centuries, perhaps even millennia.
Neither the Doctor nor Bill knew what would happen when the creature was freed. Instead of lashing out at her captors, the creature, once freed, swam away to the ocean. Despite her colossal size, she was careful not to harm anyone or collide with the bridges and buildings lining the river at South Bank. (TV: Thin Ice)
For an undetermined amount of time, the creature and her offspring were chained up under the River Thames by Sutcliffe's family. They somehow discovered that its faeces made for an ideal fossil fuel, which burned longer and hotter than coal, and decided to sell it to amass wealth and power. In order to ensure a constant supply, they would subtly influence the Frost Fairs that took place every year on the river Thames and have the offspring feed humans to their mother. (TV: Thin Ice)
During the Frost Fair of 1814, the Twelfth Doctor and Bill Potts arrived, the TARDIS having detected the creature's presence. Once they realised what was happening and that the creature was as much a victim as the people she was forced to eat, they freed her and defeated Sutcliffe. After she was freed, the creature and her offspring swam away to some northern location to live in peace. (TV: Thin Ice)
While newspaper's reported on the death of Sutcliffe, who apparently "drown[ed] in [a] snap thaw", Bill Potts was later unable to find any results via search-wise.net for "London 1814 sea creature". She also tried "serpent" and "really, really big fish". The Doctor reminded her, "Never underestimate the collective human ability to overlook the inexplicable," though also noted that the Frost Fair involved a lot of day drinking. (TV: Thin Ice)
While floating in Titan's low gravity, Bill mentioned "sea monsters in the Thames". (COMIC: The Soul Garden) When she moved into her new haunted house, Bill reassured herself that "there's no living puddles or weird robots, big fish. It's just a new house, and people you don't know. Not scary at all." (TV: Knock Knock)