Minecraft wiki entry for The_End, associated text: Endermen spawn naturally throughout the entire dimension.
The ender dragon spawns naturally on the central island when the player first arrives, but can be respawned by placing 4 end crystals, one on each side's middle block of the exit portal.
Shulkers appear within end cities on the outer islands. They do not respawn once killed, but can sometimes clone themselves when attacked by another shulker.