The Tier 12 Egg is purchasable at normal Egg Shop. It is the best Egg in Earth.
While it is worth a whopping 5,000,000,000 coins, inside the egg you will find either a Reaper, Tank, Fire King, Ice Queen, Angel, Immortous, or Mortuus. Each pet has its own strengths and weaknesses. All of them besides the Mortuus can be made gold.
Very Common: Tank