Problem/Bug Spawned Invulnerable, Persistant, Frozen, and Silent Enderdragon. Help!!

So recently I felt like getting a Boss Bar plugin but wanted to test something out, so I spawned in an Invulnerable, Persistant, Frozen, and Silent Enderdragon using: http://www.minecraftupdates.com/summon-command
to get the command. I spawned it, and it after about 5 minutes of having it, I try and delete it, i've tried /killall enderdragon, /butcher, i've even tried command block /kill @e[type=EnderDragon] and it said it worked, but didn't do it. I've tried changing the world to Peaceful in Multiverse, doesn't work, i've tried changing world to no spawning monsters in multiverse and doesn't work. I've also gone into the server.properties and changed it to peaceful and no monsters spawn and it doesn't work. I've tried everything. Please help!

 

Posted to wrong thread...removed comment.

 

Do you know where it is? Try poking it with a sword lol