Zaralok

A razor-toothed blade fin was a creature, resembling a shark, which was encountered by Amy Pond and the Eleventh Doctor on Poseidon 8.

Biology
The razor-toothed blade fin was shark-like in appearance, but had some differences between Earth sharks. It was massive in size, partially covered in armoured plates. It had two separate caudal fins. It could also survive in salt water. (VG: Shadows of the Vashta Nerada)

History
In the 23rd century, a temporal rift resulted in a razor-toothed blade fin being transplanted from its home to Earth. It attacked the underwater city Poseidon 8, smashing several passage ways. on Earth, along with a giant shark-like being. Fortunately, the Eleventh Doctor and Amy were able to defeat it by closing the rift, which pulled in the razor-toothed blade fin. (VG: TARDIS, Shadows of the Vashta Nerada)