Four 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)
2. Achieving. There are many pet battling achievements in game, and following the natural trail through the Taming [Continent] achievements up to Pandaria, and then on to the Spirit Tamers, Beasts of Fable and the Celestial Tournament was an epic journey for me and for many others. It's well worth doing. The Celestial Tournament is still, IMO, the pinnacle of PvE pet battling. However, WoD and Legion and BfA have added more battling achievements.