In 1.9, they tried to make the Enderdragon always fly toward the player, but they made a mistake and now, if the Enderdragon is summoned with commands (the one in the End will still move) it will be "idle" so it won't move from its spot, and it will also have no health bar and be invulnerable to damage, which ruins the concept. It's quite fun to fight the Enderdragon in the Overworld, where it can destroy the landscape.
Luckily I figured out a way to summon an active Enderdragon with commands. It's practically a normal, idle Enderdragon riding a bat, but since it's riding a bat, it moves around and I think it also destroys and kills things. The only problem is that it has no health bar still and is also immune to damage, but at least it moves.