Zaralok

A Zaralok was a creature resembling a giant shark. One Zaralok was encountered by Amy Pond and the Eleventh Doctor on Poseidon 8.

Biology
The Zaralok was shark-like in appearance, but had some differences to Earth sharks. It was about 50 feet long, partially covered in armoured plates, with two separate caudal fins. It could withstand the enormous pressure of the ocean floor, and could see in the dim waters there thanks to its oversized eyes. It was extremely fast and flexible. It could also survive in salt water. (VG: Shadows of the Vashta Nerada)

History
The Zaralok was bred by the Valky clan, a sea-dwelling people on the planet Shakara. The Valky clan bred the Zaraloks to fight an undersea war on their behalf but the creatures turned on their creators, eating them as well as the enemy.

In the 23rd century, a temporal rift resulted in one Zaralok 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 Zaralok as well as the Vashta Nerada back to their places of origin. (VG: TARDIS, Shadows of the Vashta Nerada)

Behind the scenes

 * While the Zaralok is never named or its origins explained in Shadows of the Vashta Nerada's character dialogue, that information is given in a "Who Fact" popup initiated by examining the first porthole on the USS Eldridge, outside which the Zaralok can be seen.