Sea creature (Thin Ice)

A creature which resembled a giant fish was imprisonned under the river Thames until 1814. (TV: Thin Ice)

Biology
The creature was massive in size, easily several times the size 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.

The Twelfth Doctor affirmed they were of terrestrial origin, perhaps from Earth's prehistoric era like the Silurians and the Sea Devils.

Diet
The creatures were carnivorous and fed on meat, mostly humans.

Abilities
The creature's presence alone seemed to cause intensely cold temperatures, enough to freeze the whole river Thames solid in winter.

The creature's feces made for an ideal fossil fuel. It could burn slower and longer than coal and could even burn underwater.

The smaller offspring hunted in groups and isolated a target before rapidly causing the ice abo to melt before pulling the target under to be fed to their mother. They detected a quarry through sound and vibrations.

Behaviour
The creatures were clearly intelligent and hunted in groups to track their prey. They would wait until their quarry was isolated before ambushing them.

However, the only reason the offspring killed humans was because their mother was imprisonned and would die of starvation without their help. Otherwise, they were reluctant to take innocent lives but did so because they were forced to. Their true nature was further proven when the mother, once freed, swam away to the ocean. Despite her colossal size, she was careful not to collide with the bridges and risk everyone on them.