Zygor

Zygor was the home planet of the Zygons. It was known to be warmer than Earth and covered in water, which was in the form of lakes with a different mineral composition than those of Earth. There were also no icecaps on the planet. Native lifeforms of Zygor included the unaltered Skarasen and the Zygons. The planet was destroyed in the war against the Xaranti, though many Zygons were able to escape (TV: Terror of the Zygons)

While posing as the Zygon that had taken her form, Elizabeth I, having presumably learned this from the Zygon, said that the Zygons lost their world when it burned in the first days of the Time War. Whether this was a new homeworld or something else is unknown. (TV: The Day of the Doctor))

Five groups of Zygons attempted to make Earth more like Zygor: those who arrived in the 12th century (TV: Terror of the Zygons), those that arrived in the 16th century and plotted to invade in the 21st century (TV: The Day of the Doctor), those that arrived in 1909 (PROSE: Sting of the Zygons), and those that arrived in the late-20th century. (HOMEVID: Zygon: When Being You Just Isn't Enough)


 * This would suggest that the destruction of Zygor probably occurred no later than 1909. Alternatively, they may have attempted to modify all of the planets they conquered.