The Ender Dragon made its first and only apperance in Scottland in Episode 311 - Cheated Dragon Attack! where it destroyed much of Scottland's main area. After trying and failing a few times, a dragon was finally spawned. The dragon destroyed everything it touched and caused quite an issue for the players. Zackandotherplayers flew with creative mode and eventually took it down.
Near the end of the episode, after the dragon was slain, Zack entered the End Portal, and in the process caused the server to crash when he took out one of the towers.
The reason that the dragon was spawned was because Zack had just backed up the server and people wanted to see how much the dragon would destroy.