Luna (Kill the Moon)

"No, no, no, no. It's never been dead. It's just taking a long time to come alive."

- The Doctor, 2049. ("Kill the Moon")

Luna (known to Humans simply as the Moon) is a moon orbiting the planet Earth. In 2049, it was discovered to actually be the egg of a giant space-borne life form, resembling a four-winged draconian butterfly.

Biology
The life cycle of the creature appears to be quite strange. The Doctor has described it and its parasites as having "unstable mass". Apparently, the egg remains dormant for millions of years at least, and only starts to show any kind of activity a few days before hatching, during which it gains extraordinarily huge amounts of mass without changing its volume and without consuming anything visibly. This increase in mass is apparently necessary for the creature to hatch from its eggshell and fly away, leaving a new egg behind it to restart the cycle.

History
According to the Doctor, the Moon has never been dead as Humans have often assumed. Rather, it has been in stasis for about a hundred million years, waiting for the time to hatch. He has speculated that the creature in the egg is unique, and there are no others of its kind in the universe.

In 2049, Luna's mass started to increase enormously (without changing its volume), provoking several disasters on Earth. Parasitic Moon Germs started to show up on its surface and the true nature of the satellite was revealed. While humanity had the choice to kill the embryo and prevent further destruction, the three Humans present on the Moon's surface at the time - Clara Oswald, Lundvik and Courtney Woods - opted to spare its life and allow it to hatch. Fortunately, the newly-born creature proved harmless and immediately laid another moon egg, identical to the one before, to replace the lost satellite. Following that, it promptly flew away into space to unknown destination.

It's not known for sure whether the moon that hatched in 2049 was the first one or not. Assuming the Doctor's estimation of it taking about 100 million years to hatch to be correct, it's likely that there have been multiple other moons before that, with the previous one hatching during the Mesozoic (perhaps witnessed by the Silurians).