Star whale

A star whale, or Galeen, was a giant, space-borne cetacean. They were sentient, and often came to the rescue of spacecraft in distress.

According to the Sixth Doctor, they were the only known animals to live in outer space. He sought to end the whaling of these creatures. (AUDIO: The Song of Megaptera) The Eleventh Doctor met the last of their kind. (TV: The Beast Below)

Biology
Star whales were sentient and were similar-looking to Earth's cetaceans in basic structure. They had several flippers and tentacular protrusions down their stomach, as well as smaller, spiked tendrils on their back.

Star whales travelled through space at speeds comparable to spaceships. (TV: The Beast Below) In order to lose a pursuer, a star whale could dive in time, using its time core to plunge beneath the event horizon. This left a trail of tachyons in its wake. (AUDIO: The Song of Megaptera)

Though they could communicate in an atmosphere, their voices were beyond the range of human hearing. (TV: The Beast Below) Galeens transmitted their song as radio waves. (AUDIO: The Song of Megaptera) They were capable of bioluminescence and had glowing patterns around their tendrils. (TV: The Beast Below)

They fed on solar power from the stars they passed, (TV: The Beast Below) using their solar sails. (AUDIO: The Song of Megaptera) They could occasionally eat humans if presented with the opportunity, but one star whale refused to harm children. (TV: The Beast Below) Generally, star whales got their nourishment from its own internal self-sustaining ecosystem. As such, the Sixth Doctor described a Galeen as "basically a micro-planet". (AUDIO: The Song of Megaptera)

Their intelligence was higher than that of humans, and they could live for thousands of years, (TV: The Beast Below, AUDIO: The Song of Megaptera) in part due to digested time particles in dark matter making their internal organs into "eternal organs". (AUDIO: The Song of Megaptera)

History
Jack Harkness believed that they originated from outside of Mutter's Spiral. (TV: Meat) 6 January was the first day of the great Star Whale migration. (PROSE: Time Traveller's Diary)

An ancient and benevolent species, star whales had, according to legend, guided early space travellers through dangers on their journeys through space. (TV: The Beast Below)

Some survivors of crashed spaceships saved by the whales even lived inside the whales for generations. (AUDIO: The Song of Megaptera)

They were extensively hunted by humans and Toothons. The Sixth Doctor and Peri saved a space whale called Megaptera from being killed by a whaling spaceship. (AUDIO: The Song of Megaptera)

One Space Whale, (PROSE: Pack Animals) degradingly nicknamed Cash Cow, was recovered from Earth's oceans as an infant and kept by members of Harries & Harries as a re-growing source of meat. This scheme was discovered and stopped by members of Torchwood Three; despite Jack Harkness' hopes to nurse it back to health, it became impossible to sedate, and Owen Harper euthanised it. (TV: Meat)

On one occasion, the Doctor's TARDIS once rested on the hump of a space whale. (WC: TARDIS Cam No.6)

One star whale swam through the Blue Lagoon Nebula, not far from where the Twelfth Doctor parked his TARDIS. (COMIC: Epilogopolis)

A few centuries before they were "all but extinct", star whales were a protected species. The Sixth Corsair was employed to capture a young star whale for the Hoarder. The Thirteenth Doctor had the Hoarder arrested by the Time Agency an she the freed the star whale. (COMIC: Old Friends)

In the 24th century, the great royal hall on the planet Hurn was constructed around the skeleton of a fully-grown star whale. (AUDIO: The Widow's Assassin)

By the 29th century, the species was believed extinct save for one individual. When Earth faced destruction because of solar flares, that star whale was drawn to Earth by the fear of the humans still there. All the peoples of Earth had escaped in massive city ships, but the people of the United Kingdom lacked an engine to get them to safety. The star whale intended to offer its services to the stranded people as it sensed the terror of the children and, being so old and kind, could not bear to see them cry.

Unfortunately, the people of the UK misinterpreted this gesture of good will as a lucky coincidence and captured it. They built their ship on its back and carved an opening to its brain to torture it into moving. The city built on its back became known as Starship UK. While the government realised the moral problem, they believed that if the whale was freed, Starship UK would be destroyed, killing the population. To deal with this, every five years they revealed the truth to citizens one at a time and allowed them to vote on the matter, choosing either to "Forget" or "Protest". Forgetting meant their memories would be erased so they didn't have to live with the guilt. However, protesting meant that particular citizen would immediately be fed to the star whale. Every year the overwhelming majority of citizens chose to forget. Similarly, Queen Elizabeth X (Liz Ten for short), whose body clock had been halted and who had ruled the British people for three hundred years, was faced with the choice to Forget or Abdicate every time she found out the truth. Abdication would free the star whale, but as previously stated, it was believed that this would destroy the Starship UK.

When the Eleventh Doctor and Amy Pond came to Starship UK in the 33rd century, they investigated the ship. Along the way, Amy stumbled into a voting booth and learned the truth, only to vote to Forget. Eventually the Doctor discovered the heart of the ship, where Hawthorne and his workers were torturing the star whale with electric shocks to keep it moving, and faced the impossible situation. Seeing no reasonable options and disgusted by what the UK government had done, the Doctor tried to render the space whale brain-dead so it would no longer feel pain from the torture, but would continue to support the ship. Amy, however, noticed the whale's neural tendrils affectionately playing with the children. She quickly put the pieces together and ran to the control system to press the "Abdicate" button with Liz Ten's hand, freeing the whale.

The ship was not destroyed as was originally feared. Instead it started to move faster. As the star whale had come willingly and was still willing to help the people of the UK, it moved more swiftly without the torture dulling its senses. Amy explained her reasoning to Liz Ten, Hawthorne and the Doctor, comparing the star whale's situation, being "very old and very kind and the last of its kind," to the Doctor himself who, like the star whale, could not bear to see children in distress. (TV: The Beast Below)

Other references
When visiting Nora Wicker in the Time Bubble, the Eleventh Doctor tells her stories about the universe, including his encounters with 'space whales.' (AUDIO: The World Tree)

When explaining her background to the Sittuun, Amy mentioned her encounter with the star whale. (PROSE: Night of the Humans) She mentioned it again on another occasion. (PROSE: The King's Dragon)

After being sent back in time by the Weeping Angels, Amy Pond used the afterword of Melody Malone's book, Melody Malone: Private Detective in Old New York Town, to tell the Eleventh Doctor to tell her younger self "she'll save a whale in outer space", as a reference to the star whale. (TV: The Angels Take Manhattan)