How can I add an entity dragon that can fly , be rideable by the player, has the ender dragon AI and Animations ?

Sadly, we can not edit vanilla elements in MCreator, but I think this is bypassable in this situation. You will most likely have to make the model of the dragon in something like Blockbench and its animations too. However, there might be no way to put these animations on without overcomplicated coding. I think you should make a separate item, craftable with the egg ("Hatchable dragon egg"). Then, make sth like "Every tick, check if the egg is going to hatch","On Hatch event replace egg with air, summon rideable dragon". You can make dragon driveable in settings as far as I know.