Sky shark

Sky sharks were a type of large, predatory sky fish.

Biology
Sky sharks looked much like the sharks of Earth. They were several metres long. Like other sky fish, they could fly through the air using electrical currents which reacted with crystalline elements in the fog. They needed this fog to survive and would die if removed from it.

They were remarkably long-lived; one sky shark in particular was encountered by a twelve-year-old Kazran Sardick, and was still alive when Kazran was an old man.

Sky sharks were attracted to sounds in the fog which resonated the crystals and created delta waves in their brains. One was attracted to the sound given off by a sonic screwdriver, going so far as to eat it. Human singing could calm the sky sharks. (DW: A Christmas Carol)

History
The sky sharks lived in the fog around Sardicktown. They were feared by the human colonists; indeed, Elliot Sardick planned to use his weather-controlling machine to keep the humans in line, using their fear to control them.

When Kazran Sardick was a child, he wanted to learn about the sky fish. The Eleventh Doctor used his sonic screwdriver to attract them. He accidentally attracted a sky shark, which bit the screwdriver in half and tried to eat Kazran and the Doctor. The Doctor got half the screwdriver and stunned the shark, but it was weakened by being removed from the fog. The Doctor planned to return the shark to sky in a cryogenic chamber to preserve it. When they went to get the chamber, the shark revived and followed them. Luckily, the chamber they chose to keep the shark in had held Abigail Pettigrew, who pacified it with her singing. The shark was returned to the clouds.

The same shark was later encountered by the Doctor, Kazran and Abigail when, on a later Christmas eve, they used it to pull a carriage, allowing them to fly through the air. Many years later, the sonic screwdriver in the shark's stomach was used to amplify and redirect Abigail's singing, allowing the fog to be controlled. Kazran and Abigail used the shark to pull their carriage on a final night together. (DW: A Christmas Carol)