The Ender Dragon can go through obsidian!
Environment
I built this massive obsidian cage for the ender dragon in my creative superflat world in the overworld. I spawned in the ender dragon with it's Spawn egg, and after a while I saw that the cage with the tiny holes which were only about 3x4 blocks. I thought everything was working, until I froze My game with /tick freeze, and then I saw the dragon outside the cage!
Duplicate of MC-5364, this is intentional.