A star whale was believed to be the last of its kind.
In the 29th century, Earth faced destruction because of solar flares, and the star whale was drawn to the planet by the fear of the humans' lives. 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. Unaware of the Star Whale's volunteering, the humans captured him and built their ship around it, and used electric shots to torture it and make it move.
The Star Whale was eventually freed after more than 300 years in space, when Amy Pond realised it was willing to drive the ship without torture. (TV: The Beast Below)