I think the Ender Dragon is an entity like the HoE, EoN, Foresight, the Wither and etc. The evidence supporting the idea:
-the ender dragon is powerfull enough to close the only exit of the end
-the ender dragon is very powerfull in general
-The ender dragon can be "resurected" by the end crystals that should not have the ability to resurect something. The main compoments of the end crystals are an eye of ender and an ghast tear. The ghast tear has the power to regenerate things and does regenerate the dragons health. While the eyes of ender are used to lead something to a certain thing (they lead you to the strongholds, you can interpretend that they lead you to the end when they are in the portal and you can interpretend that they lead the regeneration beam of the crystal to the dragon). None of the components has the power to resurect. Meaning that the body during the "resurection" is simply regenerated and that the ender dragon can exit without a physical body (like the Wither, HoE, EoN and etc).
@late shore: +when you are resurrecting the dragon you can hear several roars before the body is even created
Edit: the dragon also has basicaly a role like cerberus, but seems to be more powerfull than cerberus
